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PHOTOGRAPHY  OF  TO-DAY 


John  //.  Gear. 

A  Rainbow  from  an  Autochrome. 

The  Autochrome  itself  is  a  three-colour  photograph.  It  is  here  copied  by  the  three- 
colour  typographic  process.  This  picture  is  therefore  a  three-colour  reproduction  of  a 
three-colour  photograph. 


PHOTOGRAPHY  OF 

TO-DAY 

A  POPULAR  ACCOUNT  OF  THE  ORIGIN, \  PROGRESS 
AND  LATEST  DISCOVERIES  IN  THE  PHOTO¬ 
GRAPHER'S  ART, ;  TOLD  IN  NON¬ 
TECHNICAL  LANGUAGE 


By 

H.  CHAPMAN  JONES,  F.I.C.,  F.C.S.,  F.R.P.S. 

President  of  the  Royal  Photographic  Society  ;  Lecturer  on 
Photography  at  the  Imperial  College  of  Science 
and  Technology,  South  Kensington 


With  54  Illustrations  Diagrams 


PHILADELPHIA 

J.  B.  LIPPINCOTT  COMPANY 

LONDON  :  SEELEY,  SERVICE  &  CO.  LTD. 

I9I3 


CofJS 

nh  . 

J77 

IV3*- 


THE  GETTY  CENTER 

UBRArfY 


AUTHOR’S  NOTE 


I  have  to  thank  Mr.  John  H.  Gear  for  allowing 
his  beautiful  autochrome,  “The  Rainbow,”  to  be 
reproduced  as  the  frontispiece,  and  I  have  to 
thank  also  Messrs.  J.  H.  Dallmeyer,  Ltd.,  Dr.  H.  H. 
Hoffert,  H.M.I.,  Mr.  E.  O.  Hoppe,  Mr.  F.  V.  T. 
Lee,  The  Psychological  Review ,  Mr.  Edgar  Scamell, 
Mr.  George  Scamell,  Mr.  Arthur  E.  Smith,  The 
Thornton-Pickard  Manufacturing  Co.  Ltd.,  and  Pro¬ 
fessor  R.  W.  Wood,  for  the  characteristic  photo¬ 
graphs  that  they  have  so  kindly  contributed,  and 
in  some  cases  for  the  valuable  information  that 
they  have  freely  placed  at  my  disposal. 


CONTENTS 


CHAPTER  I 

PAGE 

Light,  Its  Nature  and  Effects . 17 

CHAPTER  II 

The  Control  of  Light . 36 

CHAPTER  III 

Lenses,  Old  and  New . 52 

CHAPTER  IV 

The  Development  of  Photography  ....  70 

CHAPTER  V 

Photography  before  Gelatine . 85 

CHAPTER  VI 

The  Use  of  Gelatine . 10 1 

CHAPTER  VII 

The  Plate . . ..  .  *115 

ix 

\ 


Contents 


chapter  VIII 

The  Exposure . 


CHAPTER  IX 


The  Development  of  the  Plate  . 


CHAPTER  X 

Finishing  the  Negative  . 


CHAPTER  XI 


Printing  in  Silver  . 


CHAPTER  XII 

Other  Printing  Methods 


CHAPTER  XIII 

Photo-mechanical  Printing 


CHAPTER  XIV 

The  Effect  of  Colour  and  its  Control 


CHAPTER  XV 

The  Photography  of  Colour  • 

CHAPTER  XVI 

Truth  and  Error  in  Photography. 


PAGE 

I3I 


149 


164 


178 


192 


211 


.  228 


245 


264 


Contents 


CHAPTER  XVII 

Instantaneous  Photography  and  the  Photography  of 
Motion . 


CHAPTER  XVIII 

On  Size  and  Scale  .... 


CHAPTER  XIX 

Sundry  Applications  of  Photography 


Index  . 


PAGE 

276 

292 

312 

337 


xi 


LIST  OF  ILLUSTRATIONS 


PLATES 

Autochrome.  A  Rainbow  ....  Frontispiece 

FACING 

PAGE 

A  Beech  Tree  in  the  New  Forest  ....  44 

Diaphragm  Distortion . 56 

West  Door  of  Beverley  Minster  .....  60 

A  Sunset  Sky  .........  76 

An  Old  Doorway  of  Marston  Trussell  Church  .  .  88 

“Paper!” . 104 

A  Vaulting  Horse . 118 

A  View  in  Mr.  Hoppe’s  Studio . 138 

A  Foot-race . 146 

Maeterlinck.  An  Example  of  Modern  Portraiture  .  160 

A  Bobsleigh  .  .  .  .  .  .  .  .  .  174 

Stonehenge,  as  seen  from  an  Aeroplane  .  .  .188 

Distortion  at  the  Edges  of  the  Plate  due  to  its 

Flatness.  A  Sphere  .......  206 

Distortion  at  the  Edges  of  the  Plate  due  to  its 

Flatness.  A  Cylinder . 206 

Photograph  of  a  Child  with  a  Fine  Screen  .  .  220 

The  same  Photograph  with  a  Coarse  Screen  .  .220 

Photographed  by  Infra-red  Light  .....  242 

Optical  Illusions.  “Life” . 25^ 

„  „  Concentric  Circles  .  .  -256 

Reticulation  .  .  27° 

Distortion  .  .  .  .  .  .  •  •  •  .270 

xiii 


List  of  Illustrations 


facing 

PAGK 


The  Life  History  of  a  Nasturtium 

„  „  »  ” . 

The  Clock  Tower,  Westminster,  by  the  Telephoto¬ 
graphic  Lens . 

The  Clock  Tower,  Westminster,  by  the  Ordinary 

Lens . 

Photomicrograph . 

>>  «  ' 

Records  of  an  Engineering  Firm  • 

»  »  »»  ” 

Lightning . 


284 

284 

294 

294 

302 

3°2 

316 

3l6 

324 


ILLUSTRATIONS  IN  TEXT 

J  PAGE 

"i.  An  Apparatus  that  serves  to  Demonstrate  the 

Fact  that  Light  takes  an  Appreciable  Time  to 

Travel  a  Few  Miles . 24 

26 

2.  A  Travelling  Movement  ..•••• 

3.  Wave  Motion .  ' 

4.  Waves  of  Different  “Wave  Lengths”  .  •  •  27 

5.  Colours  as  produced  by  Light  of  Different  Wave 

Lengths.  Illustration  of  a  Spectrum 

6.  Two  Candles  shining  upon  a  Piece  of  Cardboard  39 

7.  A  Pinhole  Screen  between  the  Candles  and  the 

Cardboard  causes  an  Image  to  be  produced  .  4° 

8.  A  Brighter  Image  produced  by  superposing  Three 

Separate  Images  .  42 

9  An  Image  produced  bv  a  Mirror,  showing  also 

the  Error  due  to  its  Spherical  Curvature  .  43 

10.  Light  passing  from  one  Medium  to  Another  con¬ 
tinues  in  a  Straight  Line  when  it  impinges 

perpendicularly  upon  the  Separating  Surface  .  46 

xiv 


List  of  Illustrations 

FIG.  PAGE 

11.  If  the  Path  of  the  Light  is  not  Perpendicular 

to  the  Separating  Surface,  its  Direction  is 

ALWAYS  CHANGED — THE  LlGHT  IS  “REFRACTED”  .  46 

12.  Three  Images  superposed  by  Means  of  Refraction  47 

13.  Illustrates  the  Essential  Difference  between 

Image-forming  Instruments — the  Telescope,  the 
Microscope,  the  Ordinary  Camera  ...  49 

14.  A  Lens  with  a  Curved  Field  cannot  give  the 

Image  on  a  Flat  Plate.  .....  54 

15.  The  Origin  of  “Curvilinear”  Distortion  .  .  56 

16.  The  Origin  of  “Curvilinear”  Distortion  .  57 

17.  Curvilinear  Distortion . 58 

18.  A  Single  Lens.  Petzval’s  Portrait  Lens.  A  Rapid 

Rectilinear  Lens . 65 

19.  Goerz’  “Double  Anastigmat.”  Zeiss’  “Protar.”  .  68 

20.  Early  Types  of  Cameras  .  .  .  .  .  -132 

21.  Watson’s  “Acme”  Camera . 136 

22.  Watson’s  “Premier”  Camera . 136 

23.  The  Earliest  Panoramic  Camera,  made  by  F.  v. 

Martens  in  1845 . .  321 


xv 


PHOTOGRAPHY  OF  TO-DAY 


CHAPTER  I 

LIGHT,  ITS  NATURE  AND  EFFECTS 

As  light  is  the  active  agent  in  photography,  it  will  be 
well  if  we  endeavour  to  find  out  something  about  its  nature 
and  what  it  is  able  to  do,  before  we  begin  the  consideration 
of  our  specific  subject.  It  has  been  suggested  that  such 
knowledge  is  unnecessary,  because  those  that  paint  do 
not  trouble  themselves  much,  if  at  all,  about  the  chemistry 
of  their  pigments  or  the  manufacture  of  their  brushes. 
Indeed,  some  would  go  further  and  argue  that  such  know¬ 
ledge  is  likely  to  be  detrimental,  because  it  tends  to  divert 
the  thoughts  of  the  worker  into  uncongenial  channels  and 
so  to  prevent  the  concentration  of  his  mind  upon  his 
picture  making,  which  is  the  chief  matter  that  should 
occupy  his  attention. 

But  although  “  pictures  ”  may  be  made  by  photographic 
means,  the  art  of  the  painter  is  not  really  comparable  to 
the  art  of  the  photographer.  The  photographer  does  not 
take  a  pencil  of  light  and  draw  with  it  as  the  “  artist  ’ 
draws  with  his  pencils,  but  he  so  regulates  his  apparatus 
that  the  light  itself  shall  do  the  drawing.  The  two  cases 
are  fundamentally  different.  The  most  important  charac¬ 
teristic  of  photography  is  that  it  is  so  largely  automatic. 
And  besides  this,  if  from  the  present  moment  all  “  picture 
making  by  means  of  photography  were  to  cease,  photo- 

17  B 


Light,  Its  Nature  and  Effects 


graphy  would  still  continue  to  be  practised  as  one  of  the 

most  useful  of  the  applied  sciences. 

It  is  sometimes  argued  that  it  is  not  necessary  o  un 
stand  the  principles  upon  which  photography  is  iounde  , 
because  it  is  sufficient  to  be  provided  with  a  kodak  or 
other  such  camera,  and  to  follow  the  instructions,  o  ge 
quite  acceptable  pictures.  It  is  equally  true  that  one  only 
needs  a  barrel  organ  or  a  musical  box  and  to  follow  the 
instructions  in  order  to  get  quite  acceptable  music.  And 
the  use  of  a  kodak  gives  its  possessor  less  insight  into 
photography  than  the  use  of  a  musical  box  can  into  the 
subject  and  practice  of  music,  so  far  as  it  is  possible  o 

compare  the  two. 

Therefore  without  further  apology  we  will  proceed  to 
discover  as  we  may  some  ideas  as  to  what  light  is  and 
how  it  acts.  If  you  stand  on  one  side  of  a  darkened  room 
looking  towards  some  one  on  the  other  side  who  strikes 
a  match,  you  will  observe  that  the  moment  he  strikes 
the  match  light  is  produced.  The  light  has  by  some  means 
moved  across  the  room,  entered  your  eyes,  and  produce^ 
an  effect  there  which  has  enabled  you  to  recognise  it. 
It  may  not  at  first  be  obvious  that  the  light  has  really 
travelled  across  the  room  in  this  case,  but  the  only  alterna¬ 
tive  is  that  something  has  gone  out  from  your  eyes  to  the 
match,  that  is  the  origin  of  the  light,  and  has  come  into 
contact  with  it.  Something  not  of  the  nature  of  an  arrow 
shot  from  a  bow,  but  of  a  long  arm  or  feeler  which  remains 
in  connection  with  the  eye  while  it  reaches  as  far  as  the 
match.  It  is  certain  that  there  has  been  communication 
between  the  two,  and  communication  is  never  possible 
between  two  things  at  a  distance  unless  a  connection  of 
some  sort  is  established  between  them.  There  is  ample 
evidence  that  nothing  is  projected  from  the  eye,  and 
accepting  this  as  proved,  the  simple  experiment  described 
demonstrates  that  light  moves  or  travels  away  from  its 
source.  It  can  be  proved  also  that  it  travels  in  all  directions 

18 


Light,  Its  Nature  and  Effects 

from  its  source  as  a  centre,  because  if  you  move  from 
corner  to  corner  of  the  room,  if  you  stand  upon  a  chair 
and  look  downwards  or  lie  on  the  floor  and  look  upwards, 
so  long  as  your  eyes  are  directed  towards  the  match,  you 
recognise  the  existence  of  the  light. 

In  an  equally  simple  way  we  can  illustrate,  if  we  cannot 
prove,  the  nature  of  this  movement.  The  kind  of  move¬ 
ment  with  which  we  are  most  familiar  is  that  of  a  railway 
train,  or  the  bullet  fired  from  a  gun.  Here  the  thing  itself 
travels  along,  and  even  if  we  could  not  see  it,  we  should 
have  proof  that  it  does  really  move  by  its  coming  into 
forceable  contact  with  anything  that  stands  in  its  \vay. 
A  soldier  does  not  see  the  bullet  that  wounds  him,  but 
he  knows  perfectly  well  that  it  has  come  from  a  distance 
and  that  it  has  come  to  him.  In  the  same  way  the  air 
travels  from  one  place  to  another  when  the  wind  blows, 
and  the  breath  from  our  lungs  travels  to  the  candle  flame 
when  we  blow  it  out. 

But  there  is  movement  of  quite  a  different  kind.  Here 
the  thing  itself  does  not  travel,  but  only  a  motion  passes 
from  one  place  to  another.  Take  two  or  three  yards  of 
rather  thick  string  or  cord  that  is  not  unduly  stiff,  as  new 
string  or  twine  is  likely  to  be,  tie  it  into  a  loop  at  one  end, 
and  hang  the  string  by  the  loop  on  a  hook  or  nail  at  the 
top  of  a  door,  or  in  a  similar  convenient  position.  Next 
take  hold  of  the  other  end  and  walk  backwards  until  the 
cord  hangs  slightly  curved,  that  is,  do  not  pull  it  too 
tight.  Now  rapidly  move  up  and  down  again  the  end 
of  the  string  that  you  hold,  or,  as  one  might  say,  give  it  a 
jerk.  This  movement  that  you  impart  to  one  end  of  the 
string  will  pass  along  the  string,  every  part  of  which  will 
in  turn  move  up  and  down,  and  if  your  vision  is  quick 
enough  you  will  see  a  hump,  or  raised  part,  travel  along 
the  string.  Indeed  you  will  easily  so  manage,  that  when 
this  up  and  down  movement  that  you  started  at  one  end 
of  the  string  gets  to  the  other  end,  the  loop  on  the  hook, 

19 


Light,  Its  Nature  and  Effects 

this  will  move  up  with  such  marked  effect  that  it  will 
jump  right  off  the  hook.  This  method  of  getting  the 
looped  end  of  a  rope  to  jump  off  a  hook  or  post  is 
commonly  practised  by  men  at  the  docks,  by  maids  with 
their  clothes  lines,  and  in  many  similar  cases.  Here  the 
rope  or  string  or  cable  does  not  travel  along,  it  is  only 

the  movement  that  travels.  , 

It  appears  to  be  necessarily  true  that  all  methods  o 
communication  between  objects  at  a  distance  from  each 
other  is  due  to  one  or  other  of  these  two  kinds  of  move- 
ment.  Either  a  material  object  travels  from  one  to  the 
other,  or  a  movement  travels.  In  the  case  of  the  string 
it  is  obvious  that  the  movement  travels  along  the  string. 
If  the  string  were  cut,  the  movement  or  the  jerk  imparted 
to  the  one  end  of  it  could  not  pass  over  the  space  between 
the  cut  ends,  however  small  it  might  happen  to  be.  How¬ 
ever  forcibly  the  one  end  might  be  jerked  the  portion 
that  hangs  on  the  hook  would  remain  perfectly  still,  I  he 
movement  of  one  part  of  the  string  cannot  impart  move¬ 
ment  to  another  part  unless  it  is  in  actual  contact  with 
it.  Therefore  we  see  that  when  a  movement  travels  there 
must  be  something  to  move,  something  that  it  can  travel 
aloncf,  and  this  something  must  be  continuous.  One  object 
cannot  affect  another  at  a  distance  unless  there  is  some 
continuous  connection  between  them,  some  substance 
along  which  the  effect  or  the  movement  can  travel  without 

interruption.1  ,  .  ,  .  , 

When  a  drum  is  struck,  the  drumhead,  is  driven  sud¬ 
denly  inwards,  and  this  movement  is  comparable  to  the 
jerk  given  to  the  end  of  the  string  just  referred  to.  We 
know  that  the  effect  of  the  stroke  travels  to  us,  because 

i  The  force  of  gravity,  by  which  is  meant  the  attraction  that  all  substances 
have  Tor  each  other  and  which  causes  them  as  far  as  possible  to  move  towards 
S  other  as  when  a  stone  falls  to  the  earth,  may  appear  to  be  an  exception 
each  other,  as  .  but  this  is  a  force  that  we  can  neither  start  nor  stop 

no/  after6  in  any  way,  and  therefore  essentially  different  from  the  forces  that 
mmediately  concern  us. 

20 


Light,  Its  Nature  and  Effects 

we  have  in  the  ear  an  arrangement  that  can  respond  to 
such  a  movement,  and  give  us  the  sensation  that  we 
call  sound.  We  are  concerned  only  with  the  fact  that 
this  effect  (sound)  travels  from  the  drum  to  us.  It  is 
clear  that  the  jerk  given  to  the  drumhead  does  not  pro¬ 
duce -an  effect  similar  to  the  result  of  the  jerk  given  to 
a  bullet  when  the  charge  behind  it  explodes,  because  there 
is  nothing  comparable  to  the  bullet  that  passes  from  the 
drum  to  us.  Here  again  it  is  the  movement  that  travels 
and  not  the  substance.  We  could  see  the  movement  travel 
along  the  string,  but  we  cannot  see  the  movement  that 
travels  from  the  drum,  because  in  this  case  it  is  the  air 
that  moves  and  through  which  the  movement  travels,  and 
the  air  is  invisible.  If  there  were  no  air  between  us  and 
the  drum,  the  sound  could  not  travel  to  us,  any  more 
than  the  movement  of  the  string  could  pass  over  an 
interval  between  the  separated  ends  of  the  string  when 
it  was  cut.  It  is  not  easy  to  remove  the  air  between  our¬ 
selves  and  a  drum  to  test  this  statement,  but  if  a  bell  is 
caused  to  ring  in  a  closed  chamber  from  which  the  air 
can  be  removed,  as  in  the  receiver  of  an  air  pump,  as 
the  air  is  pumped  out  the  sound  gets  feebler  and  feebler. 

If  you  can  find  a  continuous  long  iron  rail,  such  as 
may  be  in  a  public  park,  and  get  some  one  to  strike  it  with 
a  stone  or  a  pocket  knife  or  something  of  the  kind,  at  a 
considerable  distance  from  you  while  you  press  your  ear 
upon  the  rail,  you  will  hear  the  sound  of  the  striking  twice, 
although  the  rail  has  been  struck  only  once.  That  is 
because  the  sound  travels  through  the  iron  bar  as  well  as 
through  the  air,  and  as  it  travels  along  iron  about  fifteen 
times  more  quickly  than  through  air,  the  movement 
caused  by  the  percussion  gets  to  you  first  along  the  iron 
bar,  and  afterwards  through  the  air.  This  shows  that  the 
particular  movement  to  which  our  ears  are  sensitive 
(sound),  although  it  travels  faster  than  the  movement 
travelled  along  the  string,  takes  a  perceptible  time  to  travel 

21 


Light,  Its  Nature  and  Effects 

even  so  small  a  distance  as  twenty  or  thirty  yards.  If  a 
workman  is  watched  from  a  distance  of  about  four  hundred 
yards,  the  sound  of  each  stroke  of  his  hammer  will  not  get 
to  you  until  more  than  a  second  after  you  see  that  he  has 
made  the  stroke.  After  seeing  lightning  we  have  to  wait 
for  the  thunder,  although  they  are  both  produced  at  the 
same  time,  because  the  sound  travels  so  much  more 
slowly  than  the  light. 

We  will  return  now  to  our  first  experiment  with  the 
match.  The  light  travelled  across  the  room,  and  we  may 
ask  which  of  the  two  kinds  of  movement  took  place  ?  Was 
it  that  some  substance,  like  a  bullet  from  a  gun,  travelled 
from  the  match  to  our  eyes,  or  was  it  only  a  movement 
that  travelled  ?  It  used  to  be  considered  that  actual 
particles,  very  small  particles,  were  shot  out.  It  is  quite 
easy  to  get  particles  so  small  as  to  be  invisible  even  when 
the  utmost  power  of  the  microscope  is  at  our  disposal,  so 
that  it  is  not  so  great  a  strain  on  our  imagination  as  it 
might  at  first  be  thought  to  be,  to  suppose  that  actual 
particles  might  pass  into  our  eyes  without  injuring  them. 
But  this  theory  was  found  altogether  insufficient  to  account 
for  the  phenomena  shown  by  light,  phenomena  that  we 
cannot  discuss  here,  while  if  we  suppose  that  the  travelling 
of  light  is  simply  the  travelling  of  a  movement,  these 
difficulties  disappear. 

The  question  that  immediately  concerns  us  from  a 
photographic  point  of  view,  is  that  if  it  is  only  a  movement 
that  travels,  what  is  it  that  moves  ?  We  know  that  light 
can  travel  to  us  not  only  from  such  small  distances  as 
across  a  room,  but  from  the  sun,  which  is  more  than  ninety 
millions  of  miles  away,  and  even  from  stars  that  are  many 
thousands  of  times  as  far  off  as  the  sun.  It  is  not  con¬ 
ceivable  that  the  atmosphere  that  envelops  the  earth  can 
extend  so  far,  for  it  gets  rapidly  rarer  as  we  ascend  even  a 
few  miles  from  the  surface  of  the  earth.  And  besides,  the 
removal  of  air  to  the  utmost  extent  possible  from  a  vessel 

22 


Light,  Its  Nature  and  Effects 

in  no  way  impedes  the  passage  of  light  thiough  it.  Light 
therefore  is  not  like  sound,  a  movement  of  the  air  or  other 
ponderable  material,  but  it  is  a  movement  of  something, 
and  very  little  is  known  about  the  nature  of  the  medium 
that  moves  when  light  travels  through  it.  It  is  called  the 
luminiferous  ether,  and  supposed  to  be  a  kind  of  refined 
atmosphere  that  permeates  all  space  and  all  substances. 

We  have  seen  that  light  travels  ;  it  must  therefore  take 
time  to  travel.  If  a  cardboard  screen  is  held  in  front  of  a 
lighted  taper  on  the  other  side  of  the  room,  the  moment 
the  screen  is  removed  you  will  see  that  the  light  has 
travelled  across  the  room,  because  if  you  are  looking 
towards  it,  it  will  have  entered  your  eye  and  you  will  see  it, 
or  you  will  see  that  it  shines  upon  the  opposite  wall.  You 
will  find  it  impossible  to  detect  any  interval  between  the 
removal  of  the  screen  and  the  arrival  of  the  light  at  the  far 
side  of  the  largest  room,  or  even  at  the  greatest  distance 
out  of  doors  that  you  can  make  available  for  the  experi¬ 
ment.  But  if  instead  of  one  screen  there  are  several 
screens,  and  these  are  arranged  round  the  edge  of  a  wheel, 
so  that  they  may  be  very  quickly  moved  by  the  revolution 
of  the  wheel,  it  is  possible  to  prove  that  light  takes  a 
measurable  time  to  travel  over  a  comparatively  short 
distance. 

The  wheel  with  the  screens  round  its  edge  resembles  a 
cog-wheel,  as  shown  in  Fig.  i,  and  a  beam  of  light  can 
be  sent  through  the  space  between  one  cog  and  the  next, 
and  then  by  means  of  a  mirror  about  five  miles  away  sent 
back  to  the  wheel.  The  mirror  may  be  arranged  so  that 
the  reflected  beam  will  pass  through  the  same  opening 
that  it  shone  through  at  first.  A  person  who  looks 
through  the  opening  in  the  direction  of  the  mirror  will 
see  that  the  light  is  reflected  back  to  him,  and  the  little 
practical  difficulty  of  his  head  getting  between  the  source 
of  light  and  the  wheel  and  so  preventing  the  light  from 
shining  in  the  required  direction,  could  easily  be  overcome 

23 


Light,  Its  Nature  and  Effects 

by  means  that  we  need  not  trouble  about,  as  we  are  con¬ 
cerned  only  with  the  principle  of  the  experiment.  The 
observer,  then,  sees  that  the  light  is  reflected  back  to  him. 
If  the  wheel  is  slowly  rotated  the  light  will  be  alternately 
stopped  by  each  cog  and  allowed  to  pass  as  each  space 
between  the  cogs  comes  into  position.  The  light  that  each 
space  allows  to  pass  will  travel  to  the  mirror  and  back 
again  so  quickly  that  the  wheel  will  have  moved  an  imper¬ 
ceptible  distance  by  the  time  the  light  has  returned,  and  it 
will  still  shine  through  the  same  opening  that  allowed  it  to 
pass  to  the  mirror.  But  if  the  wheel  is  turned  with  speed 


enough,  the  observer  behind  the  wheel  will  see  no  light 
coming  from  the  mirror,  although  everything  else,  except 
the  rapid  rotation  of  the  wheel,  is  exactly  as  before.  This 
is  because  by  the  time  the  light  reflected  by  the  mirror  has 
got  back  to  the  wheel,  the  wheel  will  have  moved  sufficiently 
to  bring  a  cog  into  the  place  of  the  notch  through  which 
the  light  passed  on  its  way  to  the  mirror,  and  as  each  re¬ 
flected  beam  of  light  falls  upon  an  opaque  cog,  the  ob¬ 
server  cannot  see  any  light.  By  revolving  the  wheel  more 
quickly  still,  the  reflected  light  will  become  visible  again, 
because  each  flash  will  now  fall  upon  the  next  notch  to 
that  through  which  it  passed  at  first.  It  is  only  necessary 
to  know  the  distance  from  the  wheel  to  the  mirror,  for  this 

24 


Light,  Its  Nature  and  Effects 

doubled  will  give  the  distance  that  each  flash  of  light  has 
travelled,  the  speed  at  which  the  wheel  revolves,  and  the 
number  of  its  cogs,  to  calculate  how  long  the  light  takes  to 
make  the  journey  of  these  few  miles. 

There  are  other  methods  by  which  the  rate  at  which  light 
travels  has  been  measured,  and  they  all  agree  in  showing  that 
it  moves  through  the  air  a  distance  of  between  one  hundred 
and  eighty  and  one  hundred  and  ninety  thousand  miles  in 
one  second.  This  has  important  bearings  upon  photographic 
work.  The  most  obvious,  perhaps,  is  that  we  can  never  see 
or  photograph  an  object  as  that  object  is,  but  only  as  it  was 
when  the  light  that  reaches  us  started  from  the  object.  This 
difference  is  practically  negligible,  a  very  minute  fraction  of 
a  second,  so  far  as  terrestrial  distances  are  concerned,  but 
as  it  takes  light  about  eight  and  a  third  minutes  to  travel  from 
the  sun  to  the  earth,  we  can  never  photograph  the  sun  as  it 
is,  but  only  as  it  was  a  little  more  than  eight  minutes  before 
we  take  the  photograph.  And  some  of  the  stars  are  so  far 
off,  that  it  takes  light  hundreds  or  even  thousands  of  years 
to  travel  from  them  to  us.  A  disturbance  or  change  in 
such  a  star  that  is  seen  or  photographed  to-day,  must  have 
taken  place  long  ago,  in  some  cases  considerably  before 
the  commencement  of  the  Christian  era.  That  particular 
movement  of  the  luminiferous  ether  caused  by  the  change 
or  disturbance  has  been  travelling  ever  since,  much  as  the 
ripples  on  the  surface  of  a  lake  caused  by  the  touch  of 
a  bird  continue  to  move  towards  the  shore  after  the  bird 
has  left  the  water  and  perhaps  gone  out  of  sight.  Just  as 
our  records  of  the  sun  are  always  more  than  eight  minutes 
late,  if  we  may  so  express  it,  so  our  knowledge  of  distant 
stars  is  always  a  few  hundreds  or  thousands  of  years  after 
time,  and  we  have  no  means  of  knowing  what  has 
happened  in  the  interval,  for  no  movement  known  is  more 
rapid  than  that  of  light. 

Thus  light  is  a  travelling  movement,  and  as  it  concerns 
us  so  intimately  we  want  to  know  something  about  the 

25 


Light,  Its  Nature  and  Effects 

character  of  the  movement.  Without  discussing  the 
various  possible  kinds  of  travelling  movements,  we  will 
turn  back  to  our  first  experiment  with  the  string.  It  is 
clear  that  the  movement  travels  along  the  string,  but  a 
moment’s  thought  will  show  that  the  movement  of  each 
part  of  the  string  is  in  a  direction  across  the  line  of  the 
string.  If  the  string  were  horizontal  we  might  say  that 
each  part  of  it  in  turn  moves  up  and  down  while  the  move¬ 
ment  itself  passes  along.  Fig.  2  shows  how  the  elevated 
portion  of  the  string,  or  the  hump,  travels  along  while  the 
various  parts  of  the  string  move  transversely.  The  small 
arrows  show  the  direction  in  which  the  parts  of  the  string 
that  they  are  in  contact  with  are  moving,  and  the  dotted 


Fig.  2. — A  travelling  movement. 


line  shows  the  position  of  the  hump  at  a  little  later  stage  of 
its  travel.  This  is  the  character  of  the  movement  when 
light  travels,  and  we  shall  subsequently  see  that  it  is  of 
fundamental  importance  to  us  in  the  consideration  of 
colour  in  connection  with  photography. 

When  light  travels  there  is  never  a  single  disturbance 
as  shown  in  the  figure,  but  a  succession  of  them.  This 
may  be  illustrated,  if  the  string  is  long  enough,  by  a  suc¬ 
cession  of  jerks,  and  then  a  succession  of  humps  will  pass 
along  the  string.  Fig.  3  shows  this  “  wave  motion.  There 
are  three  possible  kinds  of  variation  in  the  character  of 
these  waves  with  which  we  have  to  do,  and  it  is  by  the 
control  of  these,  and  the  control  of  the  total  quantity  or 
bulk  of  movement  (if  this  expression  is  allowed),  that  we 
can  make  light  our  servant.  The  first  and  most  important 
of  these  is  the  “  wave  length,”  that  is  the  distance  from  one 

26 


Light,  Its  Nature  and  Effects 

crest  to  the  next,  or  from  the  top  of  one  hump  to  the  top 
of  the  adjacent  hump.  But  the  wave  length  itself  cannot 
immediately  influence  either  our  eye  or  the  photographic 
plate,  because  as  we  have  stated  before  it  is  only  that  which 
comes  into  actual  contact  with  either  the  eye  or  the  plate 
that  can  affect  it.  All  light,  whatever  its  wave  length,  travels 


Fig.  3. — Wave  motion. 


at  the  same  speed  under  the  same  conditions,  so  that  the 
shorter  the  waves  are  the  greater  must  be  the  number  of 
them  included  in  any  given  distance  ;  as  the  parent  and 
the  child  trotting  along  by  his  side  go  at  the  same  rate,  but 
the  little  one  with  shorter  legs  takes  shorter  steps,  and 
makes  up  for  this  by  taking  them  more  rapidly.  In  the 
experiment  with  the  string  each  jerk  made  a  hump  or  wave 


that  travelled  along  the  string.  If  by  any  means  these 
jerks  could  be  given  at  twice  the  rate,  the  conditions  being 
such  that  the  rate  of  movement  remains  the  same,  the 
waves  produced  would  be  half  the  length  of  the  original 
waves,  as  shown  in  Fig.  4.  It  is  clear  that  twice  the  number 
of  the  shorter  waves  as  of  the  longer  waves  will  arrive  at 
the  end  of  the  string  in  the  same  time.  Now,  although  the 
action  on  neither  the  eye  nor  the  plate  can  be  directly 

27 


Light,  Its  Nature  and  Effects 

affected  by  the  length  of  the  waves,  it  is  affected  by  the 
rapidity  with  which  the  jerks  or  impulses  are  delivered,  and 
this  as  we  see  depends  upon  the  wave  length.  The  wave 
length  is  the  cause  of  the  “frequency”  of  the  impulses, 
and  the  “  frequency  ”  is  the  immediate  cause  of  the 
character  of  the  effect  produced.  If  the  frequency  is  such 
that  the  light  appears  red,  then  a  greater  frequency  (caused 
by  a  shorter  wave  length)  will  give  light  of  an  orange  colour, 
and  by  further  increasing  the  frequency  (shortening  the 
wave  length)  we  shall  have  in  order  yellow,  green,  blue, 
and  violet  light. 

In  Fig.  5  the  comparative  wave  lengths  of  light  travel- 


3°  . 

,4° 

,  i*°  ,  

160  , 

1 7° 

.  Po 

ulna-violet 

1 

g  Z 
§  w 

>  u 

5  g 

a 

u 

infra-red 

0 

cq  2 

a 

05 

S 

0 

1  0 

Fig.  5. — Colours  as  produced  by  light  of  different  wave  lengths.  Illustration 

of  a  spectrum. 


ling  in  air  are  set  out  on  a  regular  scale  and  the  colours  are 
indicated,  and  whenever  light  is  by  any  means  separated 
into  its  constituents  or  spread  out  as  shown  here  the  result 
is  called  a  “spectrum.”  It  will  be  noticed  in  the  figure 
that  there  are  waves  too  long  and  waves  too  short  to  affect 
our  eyes,  for  the  organs  of  sight  are  of  strictly  limited 
powers.  When  light  (if  it  can  be  so  called)  that  consists  of 
waves  longer  than  the  red  or  shorter  than  the  violet  enters 
our  eyes  we  do  not  know  it,  because  our  eyes  are  not 
affected  by  such  light.  Indeed,  if  we  were  to  represent  the 
whole  range  of  light  waves  so  far  as  they  are  known  by 
extending  Fig.  5  very  considerably  indeed  both  to  the  right 
and  to  the  left,  we  should  get  some  idea  of  the  small  pro¬ 
portion  of  the  whole  that  can  affect  our  eyes,  or  in  other 
words,  enable  us  to  see.  It  is  as  if  we  had  the  whole  range 

28 


Light,  Its  Nature  and  Effects 

of  notes  as  represented  by  the  keyboard  of  a  piano,  but 
were  only  able  to  hear  the  notes  produced  by  about  one 
octave,  our  ears  being  so  made  that  while  this  octave 
produced  sound  in  them,  we  were  deaf  to  all  the  notes 
higher  and  lower.  We  shall  find  subsequently  that  light 
affects,  or  produces  changes  in,  many  things  besides  our 
eyes,  and  there  seems  to  be  no  reason  why  just  that  part  of 
the  whole  range  that  affects  our  eyes  and  so  enables  us  to 
see  should  be  the  exact  part  that  affects  other  things. 
As  a  matter  of  fact  it  is  not  so,  and  we  can  go  further  and 
say  that  probably  no  two  substances  are  exactly  alike  in 
this  matter. 

The  second  circumstance  that  affects  the  character  of 
the  movement  that  we  call  light,  is  the  amplitude  of  the 
vibrations.  If  the  moving  parts  of  the  luminiferous  ether 
move  to  a  slight  extent  only,  the  movement  is  sluggish  and 
the  effect  is  feeble,  but  if  they  move  to  a  greater  distance 
to  and  fro,  the  wave  becomes  higher,  the  movement  more 
vigorous,  and  the  effect  correspondingly  greater  or  more 
intense. 

The  third  circumstance  is  the  plane  in  which  the  vibra¬ 
tion  takes  place.  In  the  experiment  with  the  string,  the 
vibration  took  place  in  a  vertical  plane  because  we  jerked 
the  end  of  the  string  up  and  down.  But  we  might  have 
jerked  it  from  side  to  side,  and  the  vibration  would  then 
have  been  in  a  horizontal  plane,  or  it  might  have  been 
caused  to  take  place  in  any  other  plane  between  these  two. 
In  the  case  of  ordinary  light,  the  vibrations  take  place  in 
all  planes.  Light  vibrating  in  one  plane  only  is  said  to  be 
•*  polarised,”  but  this  does  not  concern  us. 

And  now  it  is  desirable  that  the  reader  should  en¬ 
deavour  to  get  as  clear  a  conception  as  possible  of  what 
ordinary  light  is,  a  clear  picture  in  his  mind  of  what  takes 
place  when  ordinary  light  travels  along  and  impinges 
upon  or  bombards  any  surface,  whether  that  is  the  retina 
of  the  eye,  the  sensitive  plate,  or  a  ripening  plum,  or  a 

29 


Light,  Its  Nature  and  Effects 

fading  curtain,  or  any  other  of  the  objects  or  substances 
on  the  face  of  the  earth  to  which  it  can  gain  access. 
Imagine  a  thin  beam  or  pencil  of  light  as  might  be  seen 
illuminating  the  dust  of  the  air  if  the  sun  were  shining 
brightly  through  a  small  round  hole  in  the  shutter  of  an 
otherwise  darkened  room.  The  various  portions  of  the 
luminiferous  ether  are  moving  to  and  fro  across  the  beam 
with  an  enormous  velocity  as  the  waves  pass  along  it,  the 
waves  are  of  an  innumerable  variety  of  lengths,  and  they 
are  moving  across  the  ray  in  all  possible  planes.  Some 
will  interfere  with  others,  for  a  portion  of  even  the  lumi¬ 
niferous  ether  cannot  move  in  two  different  directions  at 
the  same  time.  We  can  hardly  liken  it  to  an  infinite 
number  of  writhing  snakes  of  very  different  sizes,  nor  to  a 
great  number  of  strings  jerked  like  that  used  as  in  the  first 
experiment,  because  these  things  are  so  large  and  the 
movements  connected  with  light  are  so  exceedingly  small, 
and  these  things  move  so  slowly  while  in  light  the  move¬ 
ments  are  of  inconceivable  rapidity.  In  all  this  apparent 
confusion,  every  movement  is  strictly  and  absolutely 
regular,  the  confusion  is  in  our  minds  only,  and  is  the 
result  of  the  impossibility  of  picturing  to  ourselves  a 
mixture  of  such  a  vast  variety  of  simultaneous  movements. 

The  next  thing  that  we  have  to  try  to  imagine  is  what 
change,  if  any,  is  light  likely  to  produce  in  a  substance 
upon  which  it  falls.  It  may  rebound,  somewhat  after  the 
manner  of  a  ball  that  is  thrown  at  a  wall,  and  produce  no 
recognisable  effect  upon  the  object  that  it  strikes  ;  a  large 
proportion  of  it  may  pass  through  and  again  produce  no 
apparent  effect,  for  light  passes  through  glass,  quartz, 
diamond,  and  many  other  substances.  But  in  innumerable 
cases  a  change  is  produced  by  the  action  of  light,  and 
those  changes  are  of  all  kinds. 

We  have  seen  that  light  is  nothing  more  than  a  move¬ 
ment,  and  when  it  comes  into  contact  with  a  substance  it 
tends  to  impart  a  movement  to  it.  If,  in  our  first  experi- 

30 


Light,  Its  Nature  and  Effects 

ment  with  the  string,  the  string  had  been  tied  tightly  to  the 
nail  and  the  nail  were  a  little  loose  in  its  hole,  the  move¬ 
ment  that  passed  along  the  string  would  have  jerked  the 
nail  and  might  have  jerked  it  altogether  out  of  its  hole. 
Then  the  travelling  movement  would  have  been  communi¬ 
cated  to  the  nail  so  as  to  produce  a  very  obvious  change 
in  its  position  with  regard  to  the  wall.  Light  excites  or 
renders  active  substances  that  were  at  first  comparatively 
inactive.  It  may,  by  affecting  the  ether  that  permeates  the 
substance,  put  a  condition  of  strain  upon  that  substance  so 
that  it  becomes  unstable  and  therefore  changes  into  a 
different  state  that  is  more  stable  under  the  new  conditions. 
Speaking  generally,  light  acts  upon  substances  by  urging 
them  into  activity,  and  the  result  of  the  activity  will  be 
whatever  change  may  be  possible  under  the  circumstances. 
We  cannot  say  in  general  terms  that  light  tends  to  produce 
this  or  that  particular  kind  of  change,  it  is  more  true  to  say 
that  any  kind  of  change  may  result  from  its  stimulating 
influence. 

Changes  are  classified  into  two  groups,  physical  and 
chemical.  A  physical  change  takes  place  when  a  substance 
is  altered  in  form  but  remains  the  same  substance,  while 
in  a  chemical  change  the  substance  itself  is  changed  into 
one  or  more  other  substances.  Wood  may  be  cut  up 
into  sawdust — that  is  a  physical  change  because  it  is  still 
wood  \  but  if  the  wood  is  burned — that  is  a  chemical 
change,  for  the  result  of  the  burning  is  charcoal  or  ash, 
carbonic  acid  gas  and  water,  and  these  are  not  wood. 
Meat  may  be  minced,  it  may  be  chewed  ever  so  well,  and 
it  still  remains  the  same  substance ;  but  when  it  is  digested 
there  is  a  chemical  change,  for  it  is  altered  into  othei  sub¬ 
stances.  Diamond,  graphite,  and  charcoal,  although  so 
different  in  their  characters,  the  diamond  transparent  and 
hard,  the  graphite  opaque  and  soft,  differ  only  in  a  physical 
sense.  Chemically  they  are  one  and  the  same  substance, 
carbon,  for  if  an  equal  weight  of  each  is  combined  (by 

3i 


Light,  Its  Nature  and  Effects 

burning  it)  with  oxygen,  it  is  found  that  in  each  case 
exactly  the  same  weight  of  oxygen  is  required  and  exactly 
the  same  weight  of  carbonic  acid  is  produced.  If  therefore 
we  by  any  means  change  graphite  into  diamond  we  effect 
a  physical  change,  for  we  have  the  same  substance,  carbon, 
but  in  a  different  form. 

Light  can  effect  physical  changes.  If  a  solution  of 
sulphur  in  bisulphide  of  carbon  is  exposed  to  light,  the 
sulphur  is  changed  into  a  different  form  which  is  not 
soluble  in  the  liquid  and  therefore  falls  to  the  bottom 
of  the  vessel  as  a  powder.  This  insoluble  sulphur  is 
whitish,  while  ordinary  sulphur  is  yellow.  If  ordinary 
white  phosphorus  is  exposed  to  light  it  is  changed  into  red 
phosphorus,  which  is  different  from  the  original  not  only 
in  colour,  but  in  solubility,  volatility,  and  oxidisability.  It 
may  be  safely  kept  as  a  dry  powder,  while  ordinary 
phosphorus  has  to  be  kept  under  water  to  prevent  it  from 
igniting.  Selenium  is  a  substance  very  similar  to  sulphur 
in  many  of  its  properties.  When  obtained  in  the  crystalline 
form  it  allows  a  current  of  electricity  to  pass  through  it, 
though  it  offers  a  very  great  resistance  to  the  current  when 
the  selenium  is  in  the  dark.  But  when  light  shines  upon 
it,  its  resistance  is  reduced  very  greatly  indeed,  so  that 
under  otherwise  similar  conditions  it  allows  of  the  passage 
of  a  much  larger  current.  When  the  light  is  cut  off  the 
resistance  becomes  great  again.  Practical  advantage  has 
been  taken  of  this  effect  of  light  in  order  to  transmit 
photographs  by  means  of  the  telegraph — to  this  we  shall 
subsequently  refer — and  also  in  Bell’s  photophone,  which 
serves  to  change  variations  in  the  brightness  of  a  light 
into  variations  in  an  electric  current,  and  these  by  means 
of  the  telephone  are  converted  into  sound.  We  are  not 
concerned  with  these  instruments,  but  only  with  the  fact 
that  selenium  in  the  light  is  different  from  selenium  in  the 
dark,  although  it  remains  selenium  and  nothing  else  all 
the  time.  One  other  example.  Cinnabar  is  red,  and  it  is 

32 


Light,  Its  Nature  and  Effects 

a  compound  of  mercury  and  sulphur.  The  same  com¬ 
pound  of  mercury  and  sulphur  exists  also  as  a  black  sub¬ 
stance,  and  if  the  red  compound  is  put  into  an  alkaline 
liquid  and  exposed  to  light  it  changes  to  the  black  variety. 
Many  other  examples  of  such  changes  might  be  given, 
but  the  illustrations  show  that  light  is  able  to  produce 
various  kinds  of  physical  change.  We  have  seen  that  it 
can  effect  changes  in  solubility,  volatility,  oxidisability, 
electrical  conducting  power,  crystalline  form,  and  colour, 
the  substance  changed  remaining  in  all  cases  of  exactly 
the  same  composition,  that  is,  no  material  is  either  added 
to  it  or  withdrawn  from  it. 

We  will  now  look  at  a  few  cases  in  which  light  pro¬ 
duces  a  more  thorough  change,  altering  the  composition 
of  the  substance,  so  that  after  the  change  it  is  no  longer 
the  same  substance.  If  the  two  gases  hydrogen  and 
chlorine  are  mixed  in  a  glass  vessel  and  kept  in  the  dark, 
the  mixture  remains  a  mixture.  If  the  vessel  is  brought 
into  a  moderate  light,  the  two  elements  will  gradually 
combine  and  produce  hydrochloric  acid  gas.  The  mole¬ 
cules  of  a  gas  are  always  moving,  and  when  light  shines 
upon  this  mixture  the  movement  becomes  more  vigorous, 
or  it  may  be  that  a  different  kind  of  movement  is  brought 
about,  and  we  see  the  result  of  this  stimulation  in  the 
combination  of  the  gases.  If  a  more  intense  light,  such 
as  that  of  burning  magnesium,  is  caused  to  shine  upon 
the  mixture,  the  disturbance  will  be  correspondingly  greater 
and  the  combination  will  take  place  with  explosive  violence. 
The  light  here  causes  combination.  But  if  the  hydro¬ 
chloric  acid  gas  produced  is  dissolved  in  water  and  the 
solution  is  put  in  the  window,  the  disturbance  caused  by 
the  light  in  this  case  will  result  in  the  slow  decomposition 
of  the  hydrochloric  acid  and  some  of  the  chlorine  will 
be  liberated.  This  is  not  a  case  of  the  simple  separation 
of  the  hydrogen  and  the  chlorine,  because  the  hydrogen 
at  the  same  time  combines  with  oxygen  of  the  air  to  form 


Light,  Its  Nature  and  Effects 

water  ;  but  still  the  chlorine  and  the  hydrogen  are  separated 
by  the  action  of  light,  while  in  the  former  case  they  were 
caused  to  combine.  If  the  white  compound  of  chlorine 
and  silver  is  exposed  to  light,  the  compound  becomes  dark 
and  some  of  the  chlorine  separates  from  it.  If  this  experi¬ 
ment  is  done  in  a  closed  glass  tube  so  that  none  of  the 
separated  chlorine  gas  can  escape,  and  the  tube  after 
exposure  is  put  away  in  the  dark,  the  chlorine  will  gradu¬ 
ally  recombine  with  the  dark  residue  and  the  original 
white  compound  will  be  reproduced.  So  that  the  disturb¬ 
ing  action  of  the  light  in  this  case  forces  the  chlorine  off 
the  compound  against,  so  to  speak,  its  inclination,  and 
it  recombines  as  soon  as  the  disturbing  force  is  withdrawn. 
It  was  noticed  that  the  change  brought  about  in  selenium 
ceased  as  soon  as  the  light  was  withdrawn,  the  selenium 
returning  to  its  original  condition.  We  shall  find  subse¬ 
quently  some  changes  caused  by  light  that  will  continue 
to  progress  after  the  light  is  withdrawn,  and  others  that 
will  cease  altogether  when  the  light  leaves  them.  And 
there  is  no  object  here  in  multiplying  examples,  as  our 
only  aim  is  to  point  out  that  light  is  only  a  movement, 
that  it  acts  only  as  a  disturbing  force,  and  that  the  result 
of  the  disturbance  may  be  of  any  kind  according  to  the 
circumstances. 

Sometimes  light  is  supposed  to  have  a  special  tendency 
to  bleach  coloured  substances.  It  certainly  does  bleach 
coloured  curtains,  carpets,  wall  papers,  and  many  other 
things.  But  on  the  other  hand  the  green  colouring  matter 
of  the  leaves  of  trees,  the  grass,  and  vegetable  growth 
in  general,  is  produced  by  the  action  of  light.  Chloride 
of  silver  is  white  and  is  darkened  by  light,  and  innumer¬ 
able  examples  might  be  given  of  both  bleaching  and 
colour  production  by  the  action  of  light.  The  simple 
fact  is  that  light  produces  changes,  and  it  is  often  the 
case  that  the  product  of  the  change  is  different,  so 
far  as  colour  is  concerned,  from  the  original  substance. 

34 


Light,  Its  Nature  and  Effects 

It  is  not  correct  to  consider  that  light  has  any  inherent 
tendency  to  produce  changes  of  any  one  particular  kind. 
It  is  a  disturbing  or  stimulating  force,  and  the  result 
of  its  action  will  depend  upon  all  the  circumstances  of 
the  case. 


\ 


35 


CHAPTER  II 

THE  CONTROL  OF  LIGHT 


Man  has  controlled  light,  and  so  to  a  certain  extent  adapted 
it  for  his  use,  from  the  very  earliest  times.  When  the 
dweller  in  tents  pushed  open  his  tent  door  to  let  in  the 
morning  light,  and  when  he  shaded  his  eyes  with  his  hand 
in  order  to  see  more  clearly  at  a  distance,  he  modified  the 
light  so  that  it  should  the  better  serve  his  purpose.  If  the 
tent  door  had  remained  closed  there  would  have  been 
some  light  inside,  and  if  his  eyes  had  not  been  shaded  the 
distant  object  would  still  have  been  visible,  but  by  letting 
in  more  light  in  the  first  case  and  excluding  some  in  the 
second,  he  so  controlled  the  light  as  to  increase  the 
visibility  of  the  objects  that  he  wished  to  see. 

This  method  of  controlling  light  leads  us  to  the  simplest 
kind  of  photography.  Photographs  are  records  made  by 
light,  and  in  every  case  where  light  has  caused  an  obvious 
difference  in  the  parts  of  an  object  exposed  to  its  influence 
when  these  are  compared  with  the  parts  shaded  from  it, 
a  photograph  has  been  produced.  If  a  blade  of  grass 
is  pulled  up,  root  and  all,  it  will  be  seen  that  the  part  that 
was  above  the  ground  is  green  and  that  the  part  that  has 
been  buried  and  in  the  dark  is  not  green.  This  is  a  photo¬ 
graphic  record  of  how  much  was  exposed  to  light  and  how 
much  was  underground.  We  bear  upon  our  bodies  a 
photographic  record  of  the  clothes  we  wear.  Our  hands 
and  faces  are  darker  than  the  covered  parts,  and  in  the 
summer  when  the  light  is  the  most  active,  the  face  gets 
sunburnt  while  the  forehead  remains  of  a  lighter  colour 
because  it  is  shaded  by  the  hat.  The  pattern  of  lace  worn 


The  Control  of  Light 

on  a  lady’s  neck  may  be  clearly  visible  when  the  lace  is 
removed,  if  she  has  been  unduly  exposed  to  sunshine. 
White  paint  in  the  shade  turns  yellow,  while  in  the  light  it 
remains  of  its  original  colour.  The  darker  patch  behind 
a  bolt  knob  is  a  photographic  record  of  the  existence  of 
that  knob.  And  many  other  examples  might  be  given  of 
photographic  records  produced,  as  we  say,  accidentally, 
and  often  to  our  annoyance  and  inconvenience. 

There  is  a  common  idea  that  photography  has  to  do 
with  a  comparatively  small  number  of  substances,  if  not 
entirely  with  a  few  compounds  of  silver.  So  far  as  the 
number  is  restricted,  it  is  only  a  matter  of  adaptability  to 
certain  definite  ends.  A  photograph  can  be  made  on 
any  surface  that  is  affected  by  light.  A  pattern  may  be 
cut  out  in  a  suitable  material  and  placed,  for  example,  on 
the  sunward  side  of  an  apple  that  is  ripening.  In  due 
time  the  apple  will  have  a  photograph  on  it,  because  the 
sunshine  will  be  unable  to  produce  its  effect  upon  those 
parts  that  are  shielded  from  its  influence.  Devices  may 
be  produced  in  a  similar  way  by  means  of  stencil  plates 
or  cut-out  patterns  upon  innumerable  other  surfaces.  In 
these  cases  there  are  only  two  tones  or  degrees  of  darkness, 
the  one  where  the  light  has  had  free  access  to  the  surface 
and  the  other  on  those  parts  that  have  been  covered  over  ; 
but  if  instead  of  a  pattern  cut  out  of  an  opaque  material  we 
employ  a  glass  plate  with  a  deposit  on  it  of  various  degrees 
of  transparency,  a  shaded  design  may  be  produced.  In 
this  case  the  light  is  controlled  as  to  its  intensity,  or  the 
extent  of  its  action,  and  not  merely  as  to  the  place  where 
it  shall  act. 

The  method  just  described  is  the  simplest,  crudest,  and 
most  obvious  kind  of  photography.  The  impression  is 
always  of  the  same  size  as  the  original,  and  the  original 
must  be  of  such  a  character  that  it  can  be  placed  upon  and 
in  close  contact  with  the  sensitive  surface.  But  we  often 
want  a  copy  to  be  larger  or  smaller  than  the  object  itself, 

37 


The  Control  of  Light 

even  when  this  is  flat  and  of  a  character  suitable  for  giving 
a  contact  copy.  And  besides,  we  want  pictures  or  records 
of  all  sorts  of  things,  such  for  example  as  buildings, 
machines,  persons,  and  landscapes,  that  are  not  flat  and 
could  not  by  any  means  be  made  to  lie  upon  or  against 
a  sensitive  surface,  to  say  nothing  of  the  impossibility  of 
treating  them  so  on  account  of  their  size. 

In  these  cases  it  is  necessary  to  produce  an  image  of 
the  object,  getting  the  image  of  the  required  size,  and  to 
bring  this,  instead  of  the  object  itself,  into  contact  with  the 
sensitive  surface.  The  image  must  be  a  real  image,  that  is, 
one  that  can  be  received  upon  a  screen  after  the  manner 
in  which  an  optical  (or  magic)  lantern  gives  an  image  on 
the  sheet.  The  image  that  we  see  of  ourselves  in  a  looking- 
glass  is  not  a  real  image,  it  is  only  a  virtual  (or  apparent) 
image  ;  it  cannot  be  brought  into  contact  with  a  surface 
because  it  has  no  existence,  except  in  the  sense  that  it  is 
an  appearance. 

All  objects  that  we  can  see  are  visible  because  of  the 
light  that  they  give  out  and  that  travels  from  them  to  the 
eye  that  sees  them.  It  is  obvious  that  light  comes  from 
visible  things  if  they  are  self-luminous,  like  the  sun  or  a 
candle  flame,  and  it  is  just  as  true  of  objects  that  shine  by 
reflected  light,  such  as  the  moon,  the  planets,  and  all  the 
common  things  with  which  we  have  to  do.  The  problem 
that  we  have  now  to  consider  is,  how  can  this  light  be 
controlled  so  as  to  cause  it  to  give  the  image  that  we  want. 
The  great  bulk  of  light  that  emanates  from  an  object  of 
considerable  size  we  cannot  consider  en  masse ,  as  the 
attempt  to  do  so  would  lead  to  indescribable  confusion. 
And  this  is  not  only  impossible  but  unnecessary,  because 
if  we  understand  how  the  light  from  a  very  small  part  of 
the  object  can  be  manipulated  so  as  to  produce  a  corre¬ 
spondingly  small  part  of  the  image,  we  shall  have  the 
information  that  we  want,  as  it  will  only  be  necessary  to 
apply  the  same  considerations  to  everv  other  small  part  of 

38 


The  Control  of  Light 

the  object  in  order  to  understand  the  production  of  the 
complete  image.  We  may  imagine  the  object  to  be 
divided  up  in  much  the  same  manner  as  the  design  on  a 
tessellated  pavement  with  very  small  tiles  ;  then  in  order  to 
produce  an  image  the  light  that  comes  from  each  tile  must 
be  made  to  produce  a  tiny  spot  of  light  on  the  suiface  that 
is  to  receive  the  image,  each  spot  of  light  in  its  proper 
place  in  relationship  to  the  others.  We  will  consider  two 
such  small  parts,  and  in  order  to  facilitate  the  experiments 


Fig.  6. — Two  candles  shining  upon  a  sheet  of  cardboard. 


we  will  take  two  candle  flames,  for  these  will  perfectly  well 
stand  for  them. 

Take  two  lighted  candles  and  set  them  up  three  or  four 
inches  apart,  and  support  a  piece  of  white  cardboard,  a 
foot  square  or  rather  larger,  at  a  distance  of  about  eighteen 
inches  from  them,  as  in  Fig.  6.  On  darkening  the  room, 
it  will  be  seen  that  each  flame  shines  all  over  the  card  and 
there  is  no  suggestion  on  it  of  an  image  of  the  flames. 
Now  take  another  card  about  the  same  size  as  the  first, 
prick  a  hole  in  the  middle  of  it  with  a  large  pin,  and  hold 
it  half-way  between  the  two  flames  and  the  first  card. 
This  shields  the  light  from  the  card  first  set  up,  but  each 

39 


The  Control  of  Light 

flame  shines  through  the  pinhole  and  produces  a  patch  of 
light  upon  it  (Fig.  7).  Here  then  we  have  at  least  the 
elements  of  an  image.  If  we  bring  a  third  candle  near  to 
the  others,  it  also  will  give  its  patch  of  light,  and  if  we  were 
able  to  go  further  and  make  a  pattern  with  candle  flames, 
as  pyrotechnists  make  flaming  and  glowing  devices  with 
their  fireworks,  we  should  get  an  image  of  the  pattern  on 
the  screen.  As  a  matter  of  fact,  a  simple  pinhole  may  be 
quite  serviceable  for  some  practical  photographic  purposes, 
giving  a  useful  image  of  general  objects. 


Fig.  7. — A  pinhole  screen  between  the  candles  and  the  cardboard  causes 

an  image  to  be  produced. 


Now  examine  carefully  the  spots  of  light  obtained  on 
the  cardboard,  bearing  in  mind  that  each  should  be  of 
the  same  shape  as  the  flame  that  produced  it,  though  up¬ 
side  down.  That  it  should  really  be  upside  down  may 
be  demonstrated  by  raising  one  of  the  candles  and  ob¬ 
serving  that  as  it  is  lifted  up  the  spot  of  light  that  its 
flame  has  produced  moves  downwards.  The  light  travels 
in  a  straight  line,  and  if  you  like  to  represent  its  path  by  a 
knitting-needle,  you  might  thrust  it  through  the  pinhole  and 
see  how  that,  when  one  end  of  the  needle  was  raised  the 
other  end  fell,  like  a  see-saw  with  the  pinhole  for  its  support. 

40 


The  Control  of  Light 

But  what  we  specially  want  to  notice  is,  that  although 
the  flames  are  bright  the  images  are  very  dull,  and  that 
although  the  flames  have  decisive  and  sharp  edges,  the 
edges  of  the  patches  of  light  that  stand  for  the  flames  on 
the  cardboard  are  diffuse,  perhaps  so  much  so  that  it  is 
difficult  at  first  to  recognise  the  shapes  of  the  flames  in 
the  patches  of  light.  We  have  an  image,  but  it  is  dull 
and  fuzzy  instead  of  being  bright  and  sharp  as  we  want 
it  to  be.  It  is  dull  merely  because  the  hole  is  so  small 
that  very  little  light  can  get  through  it.  Make  the  hole 
larger  and  the  image  will  be  brighter,  but  it  will  be  even 
further  removed  from  sharpness.  You  may  make  another 
hole  close  to  the  first  pinhole  ;  this  will  add  its  own  quota 
of  light,  but  as  the  light  travels  in  a  straight  line,  this 
second  hole  will  give  its  own  image  by  the  side  of  the 
p  image  given  by  the  first  hole,  partly  overlapping  it,  and  so 
the  confusion  will  be  increased.  The  pinhole  is  the  first 
step  in  the  getting  of  an  image-forming  apparatus,  but  we 
i  should  be  poorly  off  if  it  could  not  be  improved  upon. 

Take  one  candle  with  the  white  card  as  before,  but 

I  in  the  intermediate  card  make  three  pinholes  in  a  row 
with  a  distance  of  about  two  inches  between  each  and 
|  the  next.  The  candle  flame  now  gives  three  spots  of 
I  light  or  images  of  the  flame  on  the  screen  because  the 
light  travels  in  a  straight  line  from  the  flame  through  each 
hole  and  onwards  to  the  white  card.  If  we  could  bring 
these  three  images  together,  so  that  they  were  all  at 

exactly  the  same  place  on  the  card,  we  should  have 

one  image  but  of  triple  brightness.  Now  the  only 
way  by  which  this  is  possible  is  by  bending  at  least 
I  two  of  the  rays.  This  is  the  essence  of  the  whole 

!  art  of  getting  a  bright  and  a  sharp  image,  namely, 

bending  the  rays  of  light  that  othevwise  would  go  astray , 
i  in  such  a  manner  that  they  shall  all  work  together  in 
the  production  of  one  image.  In  the  present  experiment 
this  can  be  done  with  two  small  pieces  of  looking-glass 


The  Control  of  Light 

held  in  the  positions  shown  in  Fig.  8.  You  will  find  it 
rather  difficult  to  hold  both  the  mirrors  still  enough  to 
get  the  three  spots  of  light  exactly  superposed,  but  the 
principle  of  the  concentration  of  the  light  will  be  clear. 

Now  imagine  a  fourth  pinhole  and  a  third  mirror,  and 
go  on  in  imagination,  for  you  cannot  do  it  experimentally, 
making  more  holes  and  putting  more  mirrors  to  bend 
the  rays  that  pass  through  them,  and  you  can  see  in  your 
mind’s  eye  the  spot  of  light  getting  brighter  and  brighter. 
We  saw  before  that  a  smaller  hole  gave  a  sharper  image 


Fig.  8. — A  brighter  image  produced  by  superposing  three  separate  images. 


though  a  duller  one,  so  fancy  now  that  all  these  holes 
are  very  small,  each  with  its  mirror,  and  we  have  not 
only  a  bright  image  but  a  sharp  image.  If  now  these 
small  holes  were  very  close  together  the  mirrors  would 
have  to  be  very  small  and  each  set  at  its  proper  angle  ; 
it  is  but  another  step  to  imagine  no  space  at  all  between 
the  holes,  and  the  tiny  mirrors  all  joined  to  form  one 
continuous  surface  of  proper  curvature  to  gather  all  the 
light  that  a  mirror  or  reflector  of  the  given  size  and  posi¬ 
tion  could  gather  and  concentrate  to  form  the  image. 

By  this  means  it  would  be  possible  to  get  a  sharp 
and  bright  image,  but  such  a  mirror  would  be  very  trouble¬ 
some  to  make,  and  could  not  be  more  than  a  mere  ring, 

42 


The  Control  of  Light 

for  otherwise  it  would  come  between  the  flame  and  the 
screen  on  which  the  image  is  desired  and  so  cut  off  the 
light  altogether.  A  curved  mirror  of  a  practical  kind  for 
giving  an  image  is  illustrated  in  Fig.  9*  und  the  full  lines 


Fig.  9. — An  image  produced  by  a  mirror,  showing  also  the  error  due 
to  its  spherical  curvature. 


drawn  from  the  candle  flame  show  how  the  light  that 
proceeds  from  it  is  reflected  and  concentrated  upon  the 
small  screen  between  the  flame  and  the  mirror.  Heie 
also  the  effective  part  of  the  mirror  is  ring-shaped, 
because  the  screen  put  to  receive  the  image  shields  the 
central  part  of  the  mirror  from  the  light.  In  the 

43 


The  Control  of  Light 

quite  early  days  of  practical  photography  mirrors  were 
sometimes  used  for  portraiture,  but  they  were  found  in¬ 
convenient  for  this  very  reason,  and  also  because  much 
stray  or  uncontrolled  light  gained  access  to  the  apparatus, 
which  of  course  had  to  open  at  the  end  directed  towards 
the  person  being  photographed.  For  astronomical  pur¬ 
poses  mirrors  offer  some  special  advantages.  Dr.  A.  A. 
Common  made  some  three  feet  in  diameter  and  one  five 
feet  in  diameter.  All  the  light  from  a  single  star  that  falls 
upon  such  a  mirror  is  concentrated  upon  a  single  point 
comparable  in  size  to  a  pinhole,  and  the  brightness  of  this 
point  or  image  may  be  millions  of  times  greater  than  the 
spot  of  light  that  a  simple  pinhole  would  give. 

The  large  mirrors  just  referred  to  were  not  exactly 
spherical  in  curvature.  The  unsuitability  of  a  truly 
spherical  mirror  when  it  extends  to  more  than  a  small 
part  of  the  sphere  of  which  it  may  be  considered  to  be 
a  portion,  is  shown  by  the  dotted  lines  in  Fig.  9.  The 
light  reflected  from  its  margins  would  not  be  sent  towards 
the  point  where  it  is  wanted  to  help  to  make  the  image 
bright.  This  fault  or  “aberration"  is  called  “spherical 
aberration,"  because  it  is  the  inevitable  result  of  using  a 
spherical  surface  for  this  purpose.  It  gradually  increases 
from  the  centre  of  the  mirror  towards  its  margins,  but  if 
the  mirror  curve  is  shallow,  being  only  a  small  part  of  the 
whole  sphere,  the  effect  of  this  aberration  is  slight  and 
an  image  of  a  useful  degree  of  sharpness  may  be  obtained. 

We  saw  in  the  first  chapter  that  ordinary  light  consists 
of  a  mixture  of  many  kinds  of  light,  light  of  many  different 
colours,  as  in  the  rainbow,  if  we  regard  the  visual  effects 
of  its  components,  or  of  many  different  wave  lengths  if 
we  regard  it  in  the  more  fundamental  way  and  irrespective 
of  our  eyes.  One  great  advantage  of  mirrors  as  image¬ 
forming  instruments  is  that  they  affect  all  these  different 
components  of  light  in  exactly  the  same  way.  There  is 
no  separation  of  them,  for  the  same  law  of  reflection 

44 


C.J. 


A  Beech  Tree  in  the  New  Forest 


The  Control  of  Light 

applies  equally  to  all.  It  is  the  same  law  that  comes 
into  play  when  an  india-rubber  ball  is  thrown  upon  the 
pavement,  or  when  a  billiard  ball  rebounds  from  the 
cushion  of  the  table  (providing  that  no  “side”  or  twist 
or  spinning  movement  is  given  to  the  ball),  namely,  that 
the  angle  at  which  it  falls  or  strikes  is  also  the  angle 
of  the  rebound,  a  law  that  probably  needs  no  further 
elucidation. 

We  have  already  seen  that  in  order  to  get  a  bright 
image  it  is  necessary  to  bend  some  of  the  divergent 
rays  of  light,  so  that  they  also  may  be  brought  to  con¬ 
tribute  to  the  formation  of  the  image.  We  have  seen 
how  these  rays  may  be  bent  by  reflection,  but  there  is 
another  and  quite  different  way  by  which  this  bending 
may  be  accomplished.  The  path  pursued  by  a  ray  or 
“pencil”  of  light  may  be  bent  by  introducing  into  its 
path  a  different  medium  from  that  in  which  it  is  travelling. 
Of  course  all  media  used  for  this  purpose  must  be  trans¬ 
parent  or  the  light  would  not  pass  through  them.  Of  the 
more  common  we  have,  besides  the  air,  water,  glass  of 
numerous  varieties,  and  many  mineral  substances  such 
as  rock-crystal  or  quartz,  Iceland  spar,  fluor  spar,  mica, 
the  diamond,  &c. 

When  a  ray  of  light  impinges  perpendicularly  upon  the 
surface  of  a  second  medium,  as  when  it  passes  from  air 
into  water  or  glass,  the  direction  of  its  path  is  not  changed, 
and  this  statement  holds  good  whether  the  surface  of  the 
second  medium  is  flat  or  curved,  as  shown  in  Fig.  io. 
The  shaded  part  may  be  taken  to  represent  a  slab  of  glass. 
But  if  the  direction  of  the  ray  is  not  perpendicular  to  the 
surface  of  the  second  medium,  then  the  direction  of  the 
path  of  the  ray  is  changed,  the  ray  becomes  bent,  and  the 
bending  so  produced  is  called,  in  optical  language,  ‘'refrac¬ 
tion,”  to  distinguish  it  from  the  bending  of  the  ray  by 
reflection.  If  the  second  medium  is  denser  than  the  first, 
as  in  the  case  of  the  passage  of  the  ray  from  air  into  glass 

45 


The  Control  of  Light 

as  shown  by  the  thick  line  in  Fig.  n,  the  path  that  the 
ray  pursues  is  bent  towards  a  line  drawn  perpendicularly  to 


Fig.  io. — Light  passing  from  one  medium  to  another  continues  in  a  straight 
line  when  it  impinges  perpendicularly  upon  the  separating  surface. 

the  surface.  In  passing  from  the  denser  to  the  rarer 
medium  the  ray  is  bent  from  the  perpendicular,  and,  if 
the  two  surfaces  of  the  glass  are  parallel,  this  second 


Fig.  ii. — If  the  path  of  the  light  is  not  perpendicular  to  the  separating  surface, 
its  direction  is  always  changed — the  light  is  “refracted.” 

• 

refraction  is  exactly  equal  to  the  first,  but  being  in  an 
opposite  direction  the  path  of  the  emergent  ray  is  parallel 
to  the  ray's  path  before  entering  the  glass ;  it  is  in  the 

46 


The  Control  of  Light 

same  direction,  but  it  is  shifted  a  little  sideways,  as  shown 
in  Fig.  ii. 

Therefore  the  net  result  of  passing  the  light  obliquely 
through  the  slab  of  glass  with  parallel  sides  is  no  per¬ 
manent  bending  or  refraction  of  the  ray,  because  what 
is  done  at  one  surface  is  undone  at  the  other.  But  when 
the  two  surfaces  of  the  glass  are  not  parallel  the  result  is 
different.  Exactly  the  same  laws  apply,  and  the  reader 
can  easily  work  out  the  result  for  himself,  but  the  effect 
is  always  that  the  direction  of  the  path  of  the  ray  is 
changed.  This  gives  us  the  second  method  of  bending 


Fig.  12. — Three  images  superposed  by  means  of  refraction. 


divergent  rays  and  concentrating  them  upon  any  required 
spot.  The  candle  flame  and  the  screen  with  its  three 
pinholes,  as  in  Fig.  8,  are  shown  again  in  Fig.  12,  but 
this  time  the  two  outer  pencils  of  light  are  bent  by  causing 
them  to  pass  through  wedge-shaped  pieces  of  glass,  and 
the  three  spots  of  light  are  superposed,  just  as  when  the 
mirrors  were  used.  A  wedge-shaped  piece  of  glass  or 
other  similar  transparent  material  is  called  a  "prism.” 

In  exactly  the  same  way  as  in  the  case  of  the  mirrors, 
we  may  imagine  that  the  pinholes  in  the  screen  are 
increased  greatly  in  number  and  that  each  has  its  appro¬ 
priate  prism  to  bend  the  ray  of  light  as  required,  and  we 

47 


The  Control  of  Light 

may  suppose  that  the  holes  are  made  smaller  to  get  a 
sharper  image.  But  in  this  case  we  can  go  further  than 
when  using  the  mirrors,  and  imagine  the  small  holes  all 
over  the  screen,  or  all  over  a  large  circular  patch  in  the 
middle  of  it,  each  with  its  own  little  prism.  If,  now,  instead 
of  all  these  tiny  prisms  a  single  piece  of  glass,  with  a 
surface  that  is  continuously  curved  as  indicated  necessary 
by  the  prisms,  is  placed  in  position,  the  card  with  its 
multitude  of  pinholes  that  we  have  produced  in  our 
imagination  may  be  removed,  and  all  the  light  that  falls 
upon  the  curved  glass  will  go  to  form  the  image.  The 
image  will  be  bright  if  the  curved  glass,  or  u  lens,”  is  large, 
because  of  the  amount  of  light  brought  to  its  formation, 
and  the  image  will  be  sharper  than  when  only  the  pinhole 
was  used,  because  we  may  regard  the  sharpness  as  the 
equivalent  of  a  much  smaller  pinhole.  Moreover  the 
apparatus  is  convenient,  because  it  is  possible  to  enclose 
all  the  space  between  the  lens  and  the  screen  that  receives 
the  image  and  so  to  shut  out  stray  light.  Thus  we  have 
realised  another  important  step  in  the  production  of  an 
apparatus  that  will  give  a  good,  useful  image,  for  we  have 
got  the  image  both  brighter  and  sharper  than  that  pro¬ 
duced  by  the  simple  pinhole,  and  we  have  obtained  these 
advantages  by  means  of  an  apparatus  that  does  not  suffer 
from  the  difficulty  of  having  to  put  the  screen  that  receives 
the  image  between  the  candle  and  the  apparatus  that 
produces  the  image. 

If  only  the  lens  would  do  exactly  what  we  have  just 
represented  it  as  doing,  it  would  be  perfect.  Unfortunately 
this  is  not  the  case,  for  a  single  piece  of  glass,  even  though 
it  were  shaped  without  any  error  as  we  have  imagined  it 
to  be,  would  suffer  from  several  imperfections  with  regard 
to  image  formation ;  it  could  never  bring  all  the  light  that 
emanates  from  each  small  portion  of  the  object  to  form 
a  similar  small  portion  of  the  image.  There  are  methods 
by  which  these  imperfections  can  be  reduced  in  amount, 

48 


The  Control  of  Light 

but  none  of  them  can  ever  be  entirely  got  rid  of,  even 
though  no  limit  were  placed  upon  the  cost  or  the  com¬ 
plexity  of  the  final  product  of  the  optician's  skill.  It  is 
not  possible  to  make  any  lens,  however  complex,  that 
will  be  at  its  best  under  all  conditions.  It  is  customary 
therefore  to  perfect  lenses  in  different  directions  according 
to  the  uses  for  which  they  are  intended.  It  is  not  fair  to 
the  optician  to  take  a  lens  made  for  one  specific  purpose 
and  apply  it  to  another,  any  more  than  it  would  be  fair  to 
a  tool-maker  to  complain  that  his  tools  were  not  efficient 


Fig.  13. — Illustrates  the  essential  difference  between  image-forming 
instruments — the  telescope,  the  microscope,  the  ordinary  camera. 


when  they  had  failed  in  the  doing  of  work  for  which  they 
were  never  intended.  In  concluding  this  chapter  we  will 
endeavour  to  get  some  idea  of  the  different  kinds  of  work 
that  lenses  are  required  for  in  photography. 

The  three  chief  classes  of  instruments  in  which  images 
are  produced  by  lenses  may  be  indicated  by  the  telescope, 
the  microscope,  and  the  photographic  camera  of  the  usual 
kind.  The  essential  characteristics  of  these  three  instru¬ 
ments  are  illustrated  diagrammatically  and  compared  in 
Fig.  13.  In  the  telescope  a  comparatively  small  image  is 


The  Control  of  Light 

produced  of  a  distant  object,  in  the  microscope  a  compara¬ 
tively  large  image  is  produced  of  a  near  and  small  object, 
and  in  both  these  cases  the  image  is  examined  through 
another  lens  that  magnifies  it.  The  only  part  of  the 
image  that  is  really  needed  is  the  central  part  that  the  eye 
can  see  through  the  lens  provided.  Here,  therefore,  the 
extent  of  the  utilised  portion  of  the  image  is  very  small, 
generally  less  and  often  very  much  less  than  an  inch  in 
diameter,  and  the  image  within  that  area  must  be  so  well 
defined  that  it  will  bear  magnification. 

But  in  the  case  of  the  ordinary  camera  the  extent  of 
the  plate  that  the  image  is  required  to  cover  is  often  very 
large.  The  most  popular  of  the  small  sizes  is  the  “  quarter- 
plate,”  and  in  order  to  cover  this  the  image  produced  must 
be  at  least  five  and  a  half  inches  in  diameter,  while  for  a 
il  half-plate  ”  it  must  be  eight  inches  in  diameter,  and 
practically  the  lens  must  give  a  larger  extent  of  image  than 
this  to  allow  of  adjustment.  These  are  but  small  plates 
such  as  amateurs  most  often  use,  and  no  definite  limit  can 
be  set  to  the  size  of  plate  that  may  be  required  for  com¬ 
mercial  purposes.  This  extent  of  required  image  is  the 
essential  characteristic  required  of  lenses  in  general  photo¬ 
graphic  practice.  Here  we  take  the  image  as  it  is.  We  do 
not  magnify  it  as  in  the  telescope  and  the  microscope,  and 
therefore  the  optician  directs  his  attention  to  the  getting  of 
a  large  area  of  image  good  as  a  whole,  rather  than  a  very 
small  area  of  image  with  such  exquisite  exactness  in  its 
details  that  it  will  stand  much  magnification  before  it 
shows  its  imperfections. 

There  is  one  other  matter  that  we  want  in  a  lens,  and 
that  is  size.  It  may  be  regarded  as  a  window,  and  the 
larger  it  is  the  more  light  it  will  admit.  With  twice  the 
amount  of  light,  a  given  amount  of  work  can  be  done  by  it 
in  half  the  time,  and  this  saving  of  time  is  often  of  the  very 
first  importance.  It  means  that  in  instantaneous  work  the 
period  of  the  exposure  may  be  halved,  and  that  the  moving 

50 


The  Control  of  Light 

object  may  therefore  be  moving  twice  as  fast  without 
causing  any  more  detriment  to  the  picture  by  its  movement. 
It  is  a  little  advantage  in  having  one’s  portrait  taken  to 
have  to  be  still  for  only  two  seconds  instead  of  four,  but 
the  rapidity  of  the  lens  is  a  matter  of  perhaps  greater  im¬ 
portance  to  the  trade  worker  than  to  any  one  else.  If  the 
time  required  for  the  exposure  is  halved,  it  means  that  the 
operator  can  do  more  work  in  the  day.  Or  if  for  any 
reason  this  should  not  be  so,  it  certainly  means  that  more 
work  can  be  got  from  one  camera  in  the  day,  and  that 
therefore  in  a  large  establishment  fewer  cameras  are 
necessary.  And  fewer  cameras  need  less  accommodation, 
less  artificial  light  when  that  is  used,  and  thus  a  general 
saving  in  workshop  expenses.  Large  and  quick  working 
lenses,  therefore,  are  not  to  be  regarded  as  fads  for  the 
eccentric  or  luxuries  for  the  rich,  for  they  may  prove  very 
profitable  even  if  they  are  very  costly. 


5i 


CHAPTER  III 

LENSES,  OLD  AND  NEW 

We  have  seen  in  a  general  way  how  lenses  act  in  the  pro¬ 
duction  of  images.  We  have  seen  too  that  a  perfect  lens 
would  be  one  that  would  collect  all  the  light  that  it  could 
according  to  its  size  from  each  point  of  the  object,  and 
concentrate  that  light  upon  the  corresponding  point  in  the 
image.  There  are  several  distinct  reasons  why  no  lens 
made  of  one  piece  of  glass  can  do  this,  however  perfect  the 
workmanship  of  it  may  be.  The  action  of  the  lens  in  each 
separate  way  that  makes  it  impossible  to  realise  the  peifect 
image  that  we  want  is  called  an  “  aberration.  An  abei  ra¬ 
tion  may  be  regarded  as  a  fault  so  far  as  the  photographer 
is  concerned,  but  it  is  not  essentially  a  fault  at  all,  because 
it  is  of  the  nature  of  the  lens  to  behave  as  it  does. 

The  aberrations  of  lenses  are  not  easy  matters  to  make 
clear  in  noil-technical  language,  but  to  pass  them  over  on 
this  account  would  lay  the  author  open  to  the  charge  of 
neglecting  one  of  the  principal  branches  of  the  subject  with 
which  this  volume  deals.  For  it  is  only  by  the  reduction 
of  these  aberrations  that  lenses  can  be  improved,  and  each 
step  in  advance  diminishes  those  restrictions  that  a 
practical  worker  with  lenses  always  feels  to  limit  his  en¬ 
deavours.  It  has  already  been  pointed  out  how  a  lens  of 
large  area,  by  giving  a  bright  image,  not  only  saves  time, 
but  enables  work  to  be  done  that  would  be  impossible  with 
a  slower  lens.  But  to  make  a  lens  of  larger  diameter  with¬ 
out  lessening  its  powers  in  other  ways,  necessitates  a  more 
thorough  correction  of  its  aberrations,  if  its  power  to  give 
a  well-defined  image  is  to  be  maintained. 

52 


Lenses,  Old  and  New 

There  is  no  aberration  that  exists  in  the  simplest  lens, 
such  as  a  spectacle  lens,  that  can  ever  be  eliminated. 
In  most  cases  the  elimination  of  an  aberration  is  theoreti¬ 
cally  impossible  with  the  various  glasses  that  are  now 
available,  but  even  if  it  were  theoretically  possible  to  get 
quite  rid  of  an  aberration,  the  fact  that  human  work  is 
never  perfect  would  set  a  limit  to  the  degree  of  perfection 
attainable.  When,  therefore,  a  lens  is  stated  to  be  “  cor¬ 
rected,”  when  it  is  stated  to  be  “  achromatic  ”  (that  is, 
without  producing  colour  effects  in  the  image),  when  it  is 
stated  to  be  “rectilinear"  (that  is,  giving  lines  in  the  image 
that  should  be  straight,  without  curvature),  it  means  no 
more  than  that  the  maker  has  set  himself  to  correct  these 
aberrations,  and  not  that  he  has  absolutely  succeeded  in 
doing  so.  It  is  much  the  same  when  we  speak  of  a  good 
boy ;  we  never  mean  a  perfectly  good  boy,  and  it  would  be 
absurd  to  reserve  the  expression  until  such  an  impossibility 
was  discovered. 

The  various  aberrations  of  lenses  all  exist  together,  and 
there  are  refinements  and  subdivisions  of  them  that  we 
cannot  pretend  to  approach.  The  complication  of  the 
character  of  the  changes  effected  by  the  lens  upon  the 
light  passing  through  it  is  past  all  powers  of  description  or 
imagination,  but  by  considering  the  more  important  aber¬ 
rations  in  their  simplest  forms  and  one  at  a  time,  it  should 
not  be  difficult  to  get  a  general  idea  of  the  nature  of  each 
of  them. 

The  field  of  a  lens  is  the  place  where  the  image  of  a 
distant  view  is  produced.  It  is  desirable  that  the  field  be 
flat,  for  one  reason  because  the  surface  of  the  sensitive 
plate  upon  which  the  image  is  required  to  be  received  is 
I  flat.  If  the  field  is  not  flat,  then  the  image  and  the  plate 
cannot  be  made  to  coincide  except  at  a  small  part,  and 
where  they  do  not  coincide  the  image  must  be  “  out  of 
focus."  This  is  illustrated  in  Fig.  14,  in  which  the  field 
is  shown  as  curved,  and  the  dotted  lines  represent  a  flat 


Lenses,  Old  and  New 

plate,  on  which  only  a  small  part  of  the  image  can  be  in 
focus  wherever  it  is  placed.  The  general  tendency  is  for 
the  field  of  the  lens  to  be  concave  as  shown,  and  it  has 
actually  been  proposed  to  use  concave  plates  like  clock 
glasses,  when  this  curvature  introduces  special  difficulties. 
For  general  work  such  plates  would  be  quite  unpractical, 


Fig.  14.— A  lens  with  a  curved  field  cannot  give  the  image  on  a  flat  plate. 


and  indeed  they  would  introduce  other  and  perhaps  more 
troublesome  irregularities.  In  photographing  spectra,  when 
a  long  narrow  sensitive  strip  is  used,  the  plate,  or  the 
flexible  film  when  that  is  used,  is  sometimes  curved  a  little 
with  considerable  advantage.  But  for  ordinary  work  and 
as  a  standard  condition,  the  flat  field  is  the  most  desirable 
and  is  invariably  aimed  at  by  the  optician.  The  difficulty 
with  this,  as  with  most  aberrations,  is  that  the  reduction  of 
it  leads  to  the  increase  of  some  other  undesirable  condi- 

54 


Lenses,  Old  and  New 

tions,  so  that  finally  it  is  a  matter  of  compromise,  and  the 
best  objective  would  not  of  necessity  be  the  one  with  the 
flattest  field,  but  that  one  which  had  all  its  residues  of 
aberrations  so  well  proportioned  that  none  was  excessive. 
In  modern  lenses  this  difficulty  is  much  reduced,  because 
of  the  greater  choice  of  material  and  the  constant  striving 
of  mathematicians  to  improve  their  formulae.  There  are 
now  available  lenses  that  have  so  nearly  a  flat  field,  that 
the  deviation  from  flatness  could  not  be  detected  in  ordi¬ 
nary  photographic  work.  The  slight  residual  want  of 

I'  flatness  is  often  of  a  more  complex  character  than  the 
simple  curve  illustrated.  It  may  for  example  be  a  double 
curve,  and  the  focus  at  the  centre  of  the  plate  may  be  a 
little  nearer  the  lens  than  the  focus  at  a  short  distance  from 
the  centre. 

Curvilinear  distortion  is  the  condition  that  leads  to  the 

I  representation  of  a  rectangular  object,  such  as  the  front  of 
a  building,  with  its  four  boundaries  bulging  out  like  the 
sides  of  a  barrel,  or  bulging  inwards  like  the  contour  of  a 

I  pincushion.  Such  distortion  is  of  course  inadmissible 
when  it  is  sufficiently  marked  to  catch  the  eye.  The  older 
photographers  considered  that  distortion  of  this  kind,  and 


A  landscape  photograph,  for  example,  with  no  telegraph 
posts  or  buildings  in  it  might  suffer  very  severely  from  this 
form  of  distortion,  and  yet  the  stranger  looking  at  the 
picture,  even  if  he  thoroughly  understood  the  matter, 
might  be  unable  to  detect  the  fault  for  the  want  of  some 
standard  of  comparison.  The  folds  of  a  lady’s  dress  may 
vary  very  greatly,  and  therefore,  if  in  a  portrait  they  are 
not  exactly  of  the  shape  that  they  happened  to  be  at  the 
time,  who  is  the  wiser,  and  what  does  it  matter  ?  But  this 
is  quite  a  wrong  attitude  to  assume.  It  might  as  well  be 
argued  that  there  is  nothing  wrong  in  deception  so  long 
as  you  are  not  found  out.  There  is  no  excuse  whatever  at 

55 


Lenses,  Old  and  New 

the  present  time  for  the  getting  of  appreciably  distorted 
images,  though  it  may  be  again  emphasised  that  the  cor¬ 
rection  of  this  aberration  is  never  absolute,  and  therefore 
a  lens  that  is  quite  satisfactory  in  an  ordinary  sense  may 
be  useless  for  surveying  by  photography,  because  here  the 
image  must  be  such  that  it  will  bear  critical  measurement 
without  revealing  any  error  above  a  certain  known  small 
maximum. 

This  distortion  is  called  curvilinear,  because  the  curving 


of  lines  that  should  be  straight  is  the  most  striking  indica¬ 
tion  of  its  presence,  but  the  name  does  not  indicate  its  real 
character  at  all.  We  have  already  seen  that  a  beam  of 
light  has  its  path  so  bent  in  passing  through  a  prism,  that 
on  its  exit  the  light  travels  in  quite  a  different  direction  from 
its  original  path.  We  have  seen  too  that  we  may  regard  a 
lens  as  a  combination  of  prisms  with  curved  surfaces.  If 
therefore  different  parts  of  the  lens  are  used  for  getting 
different  parts  of  the  image,  we  run  the  risk  of  getting  the 
beams  of  light  that  give  these  different  parts  bent  out  of 
their  true  paths.  It  is  necessary  to  have  a  diaphragm,  that 
is  a  hole  of  adjustable  size,  in  connection  with  a  lens  in 


The  eleven  candle  flames  were  placed  at  equal  distances.  The  upper  row  is  photographed  with  the  diaphragm  in  front  ot  the  lens,  and 
the  lower  row  with  the  diaphragm  behind  the  lens.  In  the  first  case  the  distances  between  the  images  gradually  decrease  toward  the  margins 
of  the  plate,  while  in  the  second  case  they  increase 


Lenses,  Old  and  New 

order  to  regulate  the  amount  of  light  that  is  admitted  to  the 
camera,  and  as  the  diaphragm  is  not  advantageously  placed 
close  to  the  lens,  we  have  in  this  arrangement  exactly  the 
circumstances  liable  to  produce  the  fault.  Fig.  15  is  a 
diagram  of  two  candles,  and  a  lens  with  a  diaphragm  that 
is  giving  images  of  the  candles  on  a  screen.  The  prism¬ 
like  action  of  the  margin  of  the  lens  bends  the  whole  beam 
of  light  that  forms  the  image  of  the  lower  candle  so  that  its 
image  is  not  formed  as  shown  by  the  dotted  lines,  but  is 
displaced  towards  the  centre  of  the  screen.  By  putting  the 


diaphragm  on  the  other  side  of  the  lens  the  displacement 
will  be  in  the  opposite  direction,  as  shown  in  Fig.  16. 

Thus  we  get  those  parts  of  the  image  that  do  not  fall 
just  on  the  centre  of  the  plate  displaced  outwards  or 
expanded,  or  displaced  towards  the  centre  or  contracted, 
the  displacement  in  either  case  increasing  in  extent  towards 
the  margins  of  the  plate.  In  order  to  illustrate  this  in  a 
practical  way,  several  lighted  candles  were  placed  in  a  row 
with  exactly  the  same  space  between  each  and  the  next. 
A  photograph  was  then  taken  with  the  diaphragm  in  front 
of  the  lens  and  another  with  the  diaphragm  behind  it.  In 
an  undistorted  photograph,  as  the  flames  are  the  same 
distance  apart,  the  images  of  them  should  be  also  the  same 
distance  apart,  but  in  these  two  photographs  which  are 

57 


Lenses,  Old  and  New 

shown  in  the  illustration  facing  page  56  measurement  will 
prove  the  fact  if  the  eye  cannot  see  it,  that  in  the  one  case 
the  images  of  the  flames  get  nearer  and  nearer  together, 
and  in  the  other  farther  and  farther  apart  as  we  go  from 
the  centre  towards  the  margins  of  the  plate. 

This  growing  expansion  in  the  one  case  and  growing 
contraction  in  the  other  is  illustrated  in  a  rather  exaggerated 
way  in  Fig.  17,  by  the  effect  that  it  would  have  upon  a 
series  of  concentric  circles  drawn  at  equal  distances  apart. 
The  central  drawing  represents  the  original  object,  the  one 
on  the  left  the  image  produced  with  the  diaphragm  in 


Fig.  i  7. — Curvilinear  distortion. 


front,  and  the  one  on  the  right  with  the  diaphragm  behind 
the  lens.  And  the  effect  upon  a  square  is  shown  at  the 
same  time.  As  the  angles  are  farther  from  the  centre  of 
the  plate  than  the  middles  of  the  sides,  they  are  displaced 
to  a  greater  extent,  and  so  the  line,  straight  in  the  original, 
is  curved  in  the  image,  curved  inwards  or  outwards  as  the 
case  may  be.  This  aberration  is  corrected  by  putting  a 
lens  on  each  side  of  the  diaphragm  so  that  the  distortion 
produced  by  each  may  be  in  an  opposite  direction,  and 
thus  a  “  rectilinear  ”  lens  is  produced.  The  lower  lens  in 
Fig.  18,  and  the  two  lenses  in  Fig.  19,  are  examples  o 
such  a  construction. 

The  nature  of  spherical  aberration  was  indicated  in  the 
last  chapter.  A  single  lens  with  spherical  surfaces  bends 

58 


Lenses,  Old  and  New 

the  light  that  passes  through  it  near  its  edges  to  an  undue 
degree  as  compared  with  the  light  that  passes  through  its 
central  parts,  so  that  each  annular  portion  of  the  lens 
passing  outwards  from  its  centre  produces  its  image  nearer 
and  nearer  to  the  lens.  This  leads  to  a  want  of  precision 
in  the  image  wherever  the  screen  may  be  placed  to  receive 
it.  It  may  be  asked,  if  spherical  surfaces  cause  this  diffi¬ 
culty  why  not  shape  them  to  more  suitable  curves  ?  The 
answer  is  that  other  curves  would  lead  to  other  difficulties, 
that  they  would  be  more  troublesome  to  produce,  and  that 
it  is  possible  to  correct  spherical  aberration  in  compound 
lenses  by  a  suitable  selection  of  the  depths  of  curvature  of 
the  various  surfaces.  There  is  here  a  wide  opportunity  for 
variation.  A  single  lens  of  a  fixed  diameter  and  fixed  thick¬ 
ness  at  its  centre,  may  have  the  necessary  curvature  divided 
between  its  two  surfaces,  or  one  surface  may  be  flat  and 
the  other  more  curved,  or  the  curved  side  may  be  made  of 
a  still  deeper  curve  and  the  flat  side  hollowed  out,  that  is, 
made  concave,  like  some  spectacle  lenses  are  now  made 
in  order  to  keep  the  glasses  away  from  the  eyelashes. 
Spherical  aberration  is  controlled  by  the  depth  of  curvature 
and  by  the  distances  between  the  components  of  compound 
lenses. 

Astigmatism  is  a  common  fault  in  the  human  eye,  and 
may  be  recognised  when  it  exists  by  looking  at  an  object 
or  drawing  like  a  wheel  with  spokes.  By  shutting  one  eye 
and  directing  the  other  to  the  centre  of  such  an  object, 
while  some  of  the  radiating  lines  will  be  seen  clearly  defined 
those  at  right  angles  to  them  will  appear  misty  or  out  of 
focus  if  astigmatism  is  present.  If  the  object  is  compara¬ 
tively  near  to  the  eye  it  may  be  possible  by  altering  its 
distance  to  get  a  clear  vision  of  the  lines  that  were  misty, 
but  those  that  were  clear  at  first  will  now  be  ill-defined. 
This  phenomenon  is  due  to  the  lens  of  the  eye  inclining 
towards  the  shape  of  an  egg  flattened  sideways  instead  of 
the  shape  of  a  flattened  ball.  The  curvature  is  of  longer 


Lenses,  Old  and  New 

radius  in  one  direction  than  in  the  other,  and  this  causes 
the  distance  at  which  a  distinct  image  is  produced  to  vary 
according  to  the  direction  of  the  lines  of  the  object. 
Astigmatism  in  the  eye  affects  the  whole  of  the  field  of 
vision,  because  it  is  due  to  the  misshapen  lens.  But  as 
the  surfaces  of  photographic  lenses  are  all  of  spherical 
curves,  there  cannot  be  any  effect  similar  to  astigmatism 
in  the  middle  of  the  plate.  On  passing  towards  the  margins, 
however,  an  effect  exactly  similar  to  that  just  described 
begins  to  be  obvious  and  increases  as  the  distance  from 
the  centre  of  the  plate  becomes  greater.  It  is  due  entirely 
to  the  obliquity  with  which  the  light  from  the  marginal  parts 
of  the  object  impinges  upon  the  lens  surface.  Astigmatism 
of  the  eyes  is  corrected  by  the  use  of  spectacles  that  have 
lenses  with  an  equal  amount  of  astigmatism  but  in  the 
opposite  direction.  In  photographic  lenses  it  is  corrected 
in  a  similar  manner,  that  is,  by  taking  care  that  some 
components  err  in  one  direction  and  some  in  the  opposite 
direction,  and  that  the  opposing  aberrations  of  the  parts 
balance  each  other. 

The  last  difficulty  that  we  will  refer  to  that  stands  in 
the  way  of  getting  a  perfect  image,  is  chromatic  aberration, 
or  the  fault  of  colour.  We  know  that  the  whole  function 
of  a  lens  is  to  bend  rays  of  light  that  are  divergent  and 
so  cause  them  to  converge  to  the  place  where  they  are 
wanted  to  increase  the  brightness  of  the  image.  But 
whenever  a  ray  of  light  is  bent  by  passing  it  through  a 
medium  different  from  that  in  which  it  is  travelling,  the 
various  components  of  which  light  in  general  consists  are 
bent  to  different  extents.  We  see  this  in  the  rainbow,  for 
this  is  produced  by  the  drops  of  rain  through  which  the 
sun  shines  bending  the  rays  out  of  their  original  course, 
and  the  effect  of  the  bending  is  the  colours  of  the  bow, 
the  blue  is  bent  more  than  the  green,  the  green  more  than 
the  yellow,  the  yellow  more  than  the  orange,  and  the 
orange  more  than  the  red. 


60 


The  Inner  Side  of  the  Central  West  Door  of 
Beverley  Minster 


c.J. 


Lenses,  Old  and  New 

This  separation  of  the  various  colours  that  are  mixed 
in  white  light  takes  place  to  a  noticeable  extent  with  a 
simple  uncorrected  lens.  If  the  lens  was  so  curved  that 
it  produced  the  image  of  a  distant  object  about  twelve 
inches  behind  the  lens,  then  the  image  produced  by  the 
blue  or  violet  rays  would  be  about  the  third  of  an  inch 
nearer  to  the  lens  than  the  image  produced  by  the  red 
rays,  and  the  images  produced  by  the  other  colours  would 
range  in  due  order  between.  So  instead  of  a  simple  image 
produced  by  the  white  light,  we  have  as  it  were  a  block 
of  images,  about  one-third  of  an  inch  thick.  If  these 
images  were  all  distinct  and  separate  the  case  would  not 
be  so  troublesome,  because  we  might  be  able  to  pick  out 
just  the  image  that  we  want.  But  the  light  that  forms 
each  image  travels  through  the  whole  space  from  the  lens 
to  the  screen,  and  therefore  any  one  of  these  coloured 
images  that  we  might  select  must  be  mixed  up  with  the 
out-of-focus  images  produced  by  all  the  other  colours,  and 
this  mixing  is  inevitable  so  long  as  white  light  is  employed. 

The  aim  of  the  optician  in  correcting  the  colour  fault 
of  lenses  is  to  bring  all  this  block  of  coloured  images 
within  a  shorter  distance,  so  that  instead  of  being  about 
the  third  of  an  inch  thick  in  a  lens  of  twelve  inches  focal 
length,  it  may  be  reduced  to,  say,  a  tenth  of  an  inch  or 
the  fiftieth  of  an  inch,  or  to  as  small  a  thickness  as  possible. 
According  to  the  principle  already  given,  it  is  never  possible 
to  reduce  this  space  through  which  the  coloured  images 
.  are  produced  to  nothing,  so  that  there  shall  be  no  separ¬ 
ation  of  the  constituents  of  the  light,  but  it  is  possible 
to  reduce  it  to  so  small  a  dimension  that  it  becomes 
negligible  in  ordinary  work.  The  method  of  correction 
is  similar  to  the  treatment  of  other  aberrations,  namely, 
by  getting  a  similar  kind  of  aberration  but  in  an  opposite 
direction,  and  combining  the  two  lenses  so  that  they 
neutralise  each  other  as  far  as  possible.  As  the  propor¬ 
tional  separation  of  the  colours  is  not  the  same  in  diffeient 

61 


Lenses,  Old  and  New 

glasses  and  other  substances  available,  they  cannot  be 
made  exactly  to  match  even  theoretically.  So  an  achro¬ 
matic  lens  is  one  in  which  those  coloured  images  that  are 
formed  nearest  to  the  lens  are  pushed  away  from  the  lens 
to  a  greater  extent  than  the  others,  in  such  a  way  that  the 
image  left  the  nearest  to  the  lens  may,  for  example,  be 
yellow,  while  the  green  is  pushed  beyond  it  to  the  same 
plane  as  the  red  image,  the  blue  image  being  still  farther 
away.  Not  more  than  two  different  coloured  images  can 
be  brought  into  exactly  the  same  place  by  this  method. 
But  by  the  use  of  a  third  correcting  element  it  is  possible 
to  get  three  of  the  different  coloured  images  to  exactly  the 
same  plane,  and  a  lens  in  which  this  is  accomplished  is 
called  li  apochromatic."  Other  things  being  equal,  it  is 
clear  that  if  three  colours  are  brought  together,  the  thick¬ 
ness  of  the  colour-block,  or  the  distance  over  which  the 
coloured  images  are  distributed,  will  be  more  reduced  than 
when  only  two  are  made  to  coincide.  It  may  thus  be 
reduced  in  thickness  to  a  fifth  or  even  a  tenth  with  an 
apochromatic  of  what  it  is  with  an  achromatic  lens. 

The  apochromatic  correction  is  met  with  chiefly  in 
microscope  objectives,  in  which  it  is  of  importance  to  get 
the  finest  images  possible,  because  they  have  to  stand  an 
enlargement  up  to  about  ten  or  it  may  be  even  twenty  or 
more  times  linear  by  means  of  the  eyepiece.  An  image 
that  is  a  tenth  of  an  inch  long  may  appear  to  be  quite 
satisfactory.  It  may  remain  good  and  sharp  when  magni¬ 
fied  up  to  half  an  inch  or  even  one  inch  in  length.  But 
when  magnified  up  to  two  inches  in  length  it  may  become 
diffused  instead  of  sharp  to  the  eye.  All  images  produced 
by  lenses  “  break  down  "  or  become  ill  defined  or  fuzzy 
if  magnified  sufficiently,  and  the  amount  of  magnification 
that  the  image  will  stand  is  a  measure  of  the  correction 
of  its  aberrations. 

Thus  with  so  many  corrections  to  make  simultaneously 
it  is  not  surprising  that  the  complete  lens,  or  objective, 

62 


Lenses,  Old  and  New 

is  often  very  complex.  A  few  types  contain  three  lenses, 
a  greater  number  contain  four,  more  still  six,  some  eight, 
and  a  few  even  more  than  eight  lenses  put  together  to  form 
what  we  understand  as  the  “lens.”  The  perfection  does 
not  of  necessity  depend  upon  the  number  of  component 
parts,  any  more  than  the  quality  of  a  manufactured  article 
depends  on  the  number  of  tools  used  in  its  construction, 
but,  other  things  being  equal,  in  both  cases  as  the  number 
increases,  the  opportunities  for  improvement  increase 
also. 

For  a  long  time  opticians  had  only  a  very  few  kinds 
of  glass  at  their  disposal.  Their  power  to  improve  optical 
instruments  was  thus  very  restricted,  but  to  attempt  to 
make  new  kinds  of  glass  was  not  a  work  to  be  lightly 
undertaken,  for  it  meant  long  and  tedious  experiments  to 
discover  the  direction  in  which  the  properties  of  the  glass 
would  be  changed  by  the  use  of  new  constituents.  How¬ 
ever,  in  1 88 1  experiments  in  this  direction  were  started  by 
Dr.  O.  Schott  assisted  by  Professor  Ernst  Abbe  in  Jena. 
Eventually  they  were  assisted  financially  by  the  German 
Government,  and  the  final  outcome  of  the  investigation 
was  the  establishment  of  the  celebrated  optical  glass  works 
at  Jena,  where  a  hundred  or  more  varieties  of  optical  glass 
may  be  obtained,  and  an  optician  who  wants  a  glass  of 
any  other  special  kind  may,  within  reasonable  limits,  rely 
upon  having  a  quantity  prepared  for  him.  It  took  opti¬ 
cians  and  mathematicians  many  years  to  make  effective 
use  of  these  opportunities  for  improving  objectives,  and 
the  firm  of  Zeiss,  who  were  pioneers  in  this  matter,  pro¬ 
posed  and  put  upon  the  market  many  forms  of  objectives 
:hat  were  afterwards  withdrawn  in  favour  of  superior 
designs.  The  new  glasses  facilitated  all  the  corrections, 
out  it  was  especially  in  the  reduction  of  chromatic  aberra¬ 
tion  and  astigmatism  that  the  chief  progress  lay. 
f  The  early  investigators  and  experimenters  in  photo¬ 
lenses  specially  made  for  the 

63 


graphic  processes  had  no 


Lenses,  Old  and  New 

purpose  of  giving  a  suitable  image  because  there  was  no  de¬ 
mand  for  such  instruments,  so  they  had  to  take  the  lenses 
that  were  to  hand  and  make  the  best  of  them.  These 
were  such  as  were  made  for  telescopes,  ordinary  achro¬ 
matic  lenses  made  of  two  lenses,  a  flint  glass  and  a  crown 
glass,  cemented  together  with  Canada  balsam.  These  lenses 
were  made  for  the  purpose  of  giving  a  good  image  over 
no  more  than  the  small  area  that  the  eyepiece  of  the 
telescope  allowed  the  observer  to  see.  When  such  a  lens 
was  used  for  the  production  of  an  image  over  a  compara¬ 
tively  large  area  its  shortcomings  were  very  obvious.  The 
definition  in  the  middle  of  the  plate  was  satisfactory,  but 
towards  the  edges  the  image  rapidly  faded  into  confusion. 
By  turning  the  lens  round  so  that  the  outward  side  be¬ 
came  the  inward  side,  the  sharpness  in  the  middle  of 
the  image  was  a  little  degraded,  but  at  the  edges  of  the 
plate  it  was  somewhat  improved.  By  mounting  it  in  a 
tube  and  fixing  a  diaphragm  at  the  outer  end  of  the  tube 
at  a  distance  about  equal  to  the  diameter  of  the  lens,  the 
defining  power  was  still  better.  The  first  lens  shown  in 
Fig.  iB  illustrates  this  construction.  The  farther  the 
diaphragm  was  removed  from  the  lens  the  flatter  was 
the  field,  or  image — a  clear  advantage  ;  but  at  the  same 
time  the  curvilinear  distortion  was  increased,  and  the  area 
of  image  produced  by  the  lens  diminished.  Within  reason¬ 
able  limits,  the  smaller  the  opening  in  the  diaphragm  the 
better  the  definition,  but  at  the  same  time  the  longer  the 
exposure  necessary.  And  this  reduction  of  the  diaphragm 
increases  the  necessary  exposure  very  rapidly,  because 
the  exposure  depends  upon  the  area  of  the  opening  that 
admits  the  light.  A  diaphragm  of  half  the  diameter  will 
necessitate  four  times  the  exposure,  as  in  reducing  the 
diameter  of  the  opening  to  one-half,  its  area  has  been 
reduced  to  one-fourth. 

The  long  exposure  made  necessary  by  the  small 
aperture  of  the  lens  was  of  very  great  importance  when 

64 


Lenses,  Old  and  New 


photography  first  became  a  practical  art,  because  the 
sensitive  material  was  then  so  much  less  sensitive  than  it  is 
now,  and  also  because  the  chief  application  of  photography 
at  that  time  was  in  the  direction  of  portraiture.  It  might 
not  matter  that  an  exposure  of  several  minutes'  duration 
was  necessary  when  photographing  a  building  or  a  land¬ 
scape,  but  it  was  out  of  the  question  to  ask  people  to  sit  or 


Fig.  i  8. — A  Single  Lens.  Petzval’s  Portrait  Lens. 

A  Rapid  Rectilinear  Lens. 

stand  still  for  such  a  time,  after  the  first  novelty  of  the 
possibility  of  getting  “sun  pictures”  had  passed  away. 

The  need  for  a  lens  of  a  larger  aperture,  and  giving 
therefore  a  more  brilliant  image,  was  obvious  to  everyone, 
but  Professor  J.  Petzval,  a  mathematician  in  the  University 
||pf  Vienna,  applied  himself  practically  to  the  question  and 
T calculated  the  portrait  lens  that  bears  his  name,  and  that  in 
\  s’Pite  of  many  competitors  holds  its  own  to  this  day.  The 
i^jdiaracter  of  this  lens  is  shown  by  the  second  drawing  in 
Jpig.  18.  The  new  lens  was  made  by  the  firm  of 

65  E 


Lenses,  Old  and  New 


Voigtlander  of  Vienna,  and  was  available  for  general  use 
about  two  years  after  the  Daguerreotype,  the  first  practical 
photographic  process,  was  made  known.  Petzval  used  two 
compound  lenses,  one  on  each  side  of  the  diaphragm, 
and  corrected  the  aberrations  so  well  that  exquisitely 
sharp  definition  was  obtained  in  the  middle  of  the  pla  e 
even  when  the  full  aperture  of  the  lens,  unreduced  by  the 
diaphragm,  was  employed.  This  lens  at  once  shortene 
the  exposure  to  about  one-eighth  or  even  less  o  ia 

previously  necessary.  ,  ^ 

But  the  new  lens  was  specially  intended  for  portraiture, 

and  was  not  very  well  adapted  for  general  views  in  which 
it  is  desirable  sometimes  to  include  a  wide  expanse  o 
country  and  show  the  detail  at  the  margins  of  the  plate 
with  very  nearly  the  same  sharpness  as  at  the  middle. 
The  older  lenses  therefore  continued  to  be  used  for  such 
work  as  this,  and  after  a  few  years  improvements  were 
effected  in  them.  Thus  there  were  the  two  sorts  of  lenses, 
portrait  lenses  and  view  lenses,  and  these  two  general 
types  remained  distinct  for  a  long  time  ;  the  one  slow  but 
giving  good  definition  over  a  large  angle  of  view,  and  the 
other  rapid  and  giving  sharp  definition  in  the  centre  of  the 

plate  but  falling  off  towards  the  margins. 

In  1866  a  lens  intermediate  between  these  two  types 
was  introduced,  and  one  that  has  enjoyed  probably  a 
greater  popularity  than  any  other.  It  has  two  similar 
compound  lenses  with  a  diaphragm  between,  as  shown 
by  the  lower  diagram  in  Fig.  18.  Steinheil  called  theml 
«  aplanats,”  because  they  would  work  satisfactorily  at  full 
aperture  ;  Dallmeyer  called  his  “rapid  rectilinears,"  because 
the  curvilinear  distortion  was  corrected;  and  Ross  in  1874 
called  his  “rapid  symmetrical, ”  because  with  the  lens  on 
either  side  of  the  diaphragm  exactly  alike,  the  construction 
was  symmetrical.  These  were  sometimes  called  rapid 
view  lenses,  they  might  equally  well  have  been  called  slow 
portrait  lenses.  This  kind  of  lens  was  made  of  very 

66 


Lenses,  Old  and  New 

various  degrees  of  rapidity,  and  so  the  old  distinction 
between  the  portrait  and  the  view  lens  gradually  passed 
away.  There  are  now  made  lenses  far  more  rapid  than 
the  original  Petzval  lens,  and  we  may  pass  by  almost 
insensible  steps  to  lenses  as  slow  as  any  old  view  lens. 
Those  with  the  smaller  apertures  gain  correspondingly  in 
other  directions.  The  applications  of  photography  are 
now  so  numerous  that  they  can  no  longer  be  summed  up 
in  the  two  words — portraits  and  views. 

The  last  revolutionary  progress  in  the  improvement  of 
ordinary  photographic  lenses  took  place  about  twenty 
years  ago,  and  was  the  result  of  the  application  of  the  new 
optical  glasses  made  at  Jena.  All  the  lenses  referred  to  in 
detail  above  suffered  to  a  notable  extent  from  astigmatism 
and  curvature  of  field,  and  these  aberrations  had  to  be 
reduced,  when  they  became  obtrusive,  by  the  use  of  a 
small  diaphragm,  with,  of  course,  the  disadvantage  of  a 
longer  exposure.  Many  of  the  new  glasses  were  made  for 
the  purpose  of  facilitating  the  correction  of  these  aber¬ 
rations.  Several  lenses  were  designed  and  many  actually 
put  on  the  market,  but  the  first  that  gained  a  notable 
degree  of  appreciation  was  the  double  anastigmat  of  Goerz, 
calculated  by  Dr.  E.  von  Hoegh,  which  was  introduced  in 
1893.  (See  the  left  hand  diagram  in  Fig.  19.)  Within  a 
very  few  years  almost  all  opticians  of  note  had  issued 
mastigmats  as  calculated  by  their  mathematicians,  varying 
/erY  much  in  their  general  characteristics  and  con¬ 
duction,  but  all  alike  in  being  considerable  improvements 
)n  the  older  types.  One  of  the  most  highly  appreciated 
|s  the  lt  protar  ”  of  Zeiss,  the  right  hand  lens  shown  in 
dg.  19.  The  chief  characteristic  of  these  modern  lenses 
v  that  they  give  good  definition  at  full  aperture  over  a 
ery  much  larger  area  of  the  plate,  other  details  being 
be  same. 

Suppose  that  it  were  possible  to  make  a  lens  quite 
'ee  from  all  faults,  its  aberrations  being  absolutely 

67 


Lenses,  Old  and  New 

corrected,  with  a  very  large  aperture  and  giving  good 
definition  all  over  a  very  large  field,  its  practical  use 
would  be  severely  limited  because  of  the  inherent 
characteristics  of  all  lenses  irrespective  of  their  aberra¬ 
tions.  The  image  of  each  object  is  formed  by  every 
lens  at  a  definite  distance  from  it,  and  for  the  same  lens 
this  distance  depends  on  the  distance  of  the  objec  . 
a  telescope  is  focussed  on  an  object  that  is  a  lon£  way 
off  and  then  pointed  to  a  house  on  the  other  side  of  the 
road,  it  must  be  refocussed  to  see  clearly  the  details  of 
the  house,  because  the  image  of  the  nearer  house  is 


formed  farther  behind  the  lens  than  the  image  of  the 
object  at  a  distance.  It  is  exactly  the  same  in  the  use 
of  a  camera.  If  the  church  or  village  at  a  considerable 
distance  is  focussed,  the  figure  standing  three  or  four 
yards  away  will  be  out  of  focus,  and  if  the  figure  is 
focussed  the  other  will  be  ill  defined.  But  if  both  are 
required  to  form  parts  of  the  same  picture  they  must  both 
be  got  on  the  same  plate  at  the  same  time  and  they  must 
both  be  passably  well  defined.  The  only  way  to  secure 
this  is  to  reduce  the  aperture  of  the  lens.  Now  as 
reducing  the  aperture  reduces  all  the  aberrations 
affect  defining  power,  it  may  be  that  with  subjects  tha 
impose  such  limitations,  one  of  the  older  lenses  will  give 
just  as  good  a  result  as  the  finest  of  modern  anastigmats. 


Lenses,  Old  and  New 

On  this  account,  and  also  for  economic  reasons,  the 
older  lenses  are  still  extensively  used.  Even  single 
uncorrected  lenses,  mere  spectacle  lenses,  are  sometimes 
employed,  for  if  the  camera  and  its  adjuncts  complete  are 
to  be  sold  for  five  shillings,  the  lens  must  be  very  cheap. 
In  such  a  case  one  must  be  satisfied  with  a  small  diaphragm 
and  the  equivalent  length  of  exposure,  definition  that  is  not 
of  the  finest,  and  other  drawbacks  that  will  be  referred  to 
later.  Although  anastigmats  can  now  be  obtained  at  a 
very  reasonable  cost,  the  older  “rapid  rectilinears ”  are 
very  much  cheaper,  and  therefore  they  are  still  used  when 
cost  is  a  consideration.  The  older  lenses  are  none  the 
worse  because  of  the  introduction  of  superior  instruments. 
Excellent  work  was  done  with  them  when  nothing  better 
was  to  be  had,  and  just  as  excellent  work  may  be  done 
with  them  still. 


\ 


CHAPTER  IV 

the  development  of  photography 

IN  order  to  appreciate  properly  the  position  that  photo¬ 
graphy  occupies  at  the  present  day,  it  is  necessary  to  have 
a  general  idea  ot  the  steps  by  which  it  lias  come  to  be 
what  it  is.  We  sometimes  have  such  questions  put  to  us 
as—Who  invented  photography?  Who  discovered  photo¬ 
graphy  ■>  Who  was  the  first  photographer  ?  The  endeavour 
fo  answer  these  questions  has  led  to  waste  of  words  and 
sometimes  to  loss  of  temper,  for  those  engaged  in  the 
discussion  have  failed  to  settle  among  themselves  what 
photography  is.  is  photography  to  date  from  the  firs 
attempt  to  produce  a  design  by  light  action,  the  first 
attempt  to  get  a  permanent  record,  the  first  successful 
attempt,  or 'the  working  out  of  the  first  method  that 
« caught  on”  with  the  general  public?  Until  such 
questions  are  answered  we  cannot  say  who  was  first.  It 

is  a  matter  of  definition.  #  , 

But  if  we  look  at  the  question  broadly,  we  find  that 
photography  or  light-writing  is  older  than  man  fnmse. 
Before  the  earth  was  fit  for  him  to  live  on  the  light  was  a 
work  and  we  can  still  read  some  of  its  ancient  records 
in  fossilised  vegetable  remains.  In  the  oldest  of  histone 
literature  there  are  references  to  coloured  textile  fabrics, 
and  it  is  impossible  to  imagine  that  the  action  of  hgh 
upon  such  materials  was  unobserved.  It  is  not  known 
how  old  is  the  practice  of  exposing  linen  on  meadow  lan 
to  the  free  action  of  air,  moisture,  and  light  for  the  purpose 
of  bleaching  it.  Those  employed  in  this  work  can  hard  y 
have  failed  to  notice  that  a  shaded  portion  was  changed  t 

70 


The  Development  of  Photography 

a  much  less  extent  than  a  part  exposed  to  the  full  light, 
and  this  difference  would  constitute  a  light  record  of  the 
fact  that  a  part  was  shaded,  and  such  a  record  would 
naturally  be  accepted  as  a  guide  for  the  treatment  or  care 
of  the  material.  It  may  be  objected  that  if  this  is  called 
photography  then  a  mere  blot  of  ink  must  be  called  a 
writing.  We  will  not  argue  as  to  whether  intelligibility  is 
a  necessary  attribute  of  all  writings,  nor  refer  to  human 
intervention,  nor  consider  the  connection  of  motive  with 
the  matter,  but  endeavour  to  give  a  concise  account  of 
those  circumstances  that  are  immediately  connected  with 
our  subject. 

We  have  already  seen,  and  shall  see  more  in  detail 
subsequently,  that  the  camera  is  not  of  fundamental 
importance  in  photography.  But  as  it  has  been  and  even 
still  is  often  considered  to  include  the  lens,  the  camera 
and  lens  being  regarded  together  as  the  image-producing 
instrument,  the  history  of  the  camera  is  not  without 
interest.  In  countries  where  the  sunshine  is  much  more 
brilliant  than  we  in  England  are  accustomed  to,  and  where 
its  very  brilliancy  leads  to  the  necessity  of  excluding  it, 
there  must  often  have  been  noticed  an  image  of  exterior 
objects  on  the  wall  of  a  room  opposite  a  keyhole  or  chink. 
Major-General  Waterhouse  states  that  in  India  he  often 
saw  vivid  pictures  so  produced  on  the  wall  of  his 
bungalow.  This  is  the  very  essence  of  the  camera.  We 
only  want  a  bigger  hole  to  let  in  more  light  and  a  lens 
or  mirror  to  get  a  sharp  image  and  the  apparatus  is 
complete. 

Roger  Bacon  (born  1214,  died  1284)  was,  so  far  as 
known,  the  first  to  refer  specifically  to  such  an  apparatus, 
though  the  use  of  image-producing  lenses  and  mirrors 
was  known  more  than  a  hundred  years  before  he  was 
born.  Bacon  refers  only  to  the  use  of  a  mirror  or 
j  speculum  for  producing  the  picture.  The  first  reference 
to  the  use  of  a  lens  for  this  purpose  appears  to  have  been 

71 


The  Development  of  Photography 

made  in  1568  by  Daniello  Barbaro  in  a  book  on  per¬ 
spective  published  in  Venice,  who  directs  that  an  old 
man’s  glass  convex  on  both  sides,  not  concave,  like  the 
glasses  of  youths  with  short  sight,  ’  should  be  fixed  in  a 
hole  in  the  window,  and  all  light  stopped  out  except  what 
comes  in  through  the  lens.  A  piece  of  papei  is  held  at 
the  most  suitable  distance  from  the  lens,  and  the  image, 
he  says,  will  be  better  if  the  lens  is  partially  covered  so 
that  only  the  central  portion  is  used.  Giovanni  Baptista 
della  Porta,  who  lived  in  the  sixteenth  century,  has  often 
been  stated  to  have  invented  the  camera,  but  he  appears 
only  to  have  popularised  it  and  given  rather  more  par¬ 
ticular  instructions  concerning  it,  doubtless  introducing  a 
few  minor  improvements  in  its  details.  1  he  aim  of  these 
and  numerous  other  of  the  earlier  users  of  cameras,  was 
either  the  amusement  of  spectators,  or  to  illustrate  the 
rules  of  perspective,  or  else  to  facilitate  the  drawing  of  the 
object  depicted.  There  could  have  been  no  thought  of 
the  application  of  the  apparatus  to  photography,  because 
there  was  not  available  any  suitable  sensitive  mateiial  to 
receive  the  image  on.  To  them  the  very  idea  of  it,  if  it 
had  been  suggested,  would  have  appeared  as  wild  and 
impossible  as  the  use  of  steam  instead  of  horses  for  the 
purposes  of  locomotion,  or  the  use  of  the  same  steam 
power  instead  of  candles  and  lamps  for  the  illumination 
of  their  dwellings. 

At  about  the  same  time  that  Porta  and  Barbaro  lived  the 
alchemists  were  still  at  work,  seeking  in  their  enigmatical 
methods  to  find  a  way  of  turning  base  metals  into  gold 
and  to  find  a  method  of  curing  all  human  ills.  Silver 
chloride,  called  horn  silver,  was  known  to  them  as  a 
mineral,  and  there  is  some  evidence  that  they  were  aware 
that  it  became  darkened  or  blackened  when  exposed  to  the 
light.  It  is  difficult  to  believe  that  this  was  not  known 
long  before,  as  Geber,  an  Arabian  who  lived  in  the  eighth 
century,  prepared  nitric  acid,  dissolved  silver  in  it  and 

72 


The  Development  of  Photography 

obtained  silver  nitrate  in  crystals.  He  added  sal  ammoniac 
(ammonium  chloride)  to  the  nitric  acid  and  so  obtained 
aqua  regia,  with  which  he  dissolved  gold  and  other  sub¬ 
stances,  and  he  says  that  he  dissolved  silver  also  in  it. 


When  aqua  regia  acts  on  silver,  silver  chloride  is  produced. 
I  Or  if,  as  is  very  likely,  he  added  sal  ammoniac  to  the  solu¬ 
tion  of  the  silver  in  nitric  acid,  silver  chloride  would  have 
|  been  precipitated  as  a  white  insoluble  powder.  It  would  be 
improbable  that  experiments  of  this  kind  should  have  been 
j  made  in  daylight  without  noticing  that  the  silver  salt 
darkened  in  the  light.  But  there  was  no  reason  why 
special  notice  should  have  been  taken  and  recorded  of  this 
fact,  any  more  than,  for  example,  that  silver  was  a  white 
metal  and  copper  red. 

In  1727  a  German  physician,  Dr.  John  Hermann  Schulze, 
of  many  and  various  attainments,  for  he  was  in  turn  pro¬ 
fessor  of  anatomy,  Greek,  Arabic,  eloquence  and  anti¬ 
quities,  and  a  historian  of  ancient  medicine,  while  reading 
about  a  certain  phosphorescent  substance,  took  it  into  his 
head  to  try  to  make  some.  The  first  operation  in  its 
preparation  consisted  in  adding  nitric  acid  to  chalk.  He 
took  some  acid  that  he  happened  to  have  handy,  probably 
t  had  been  used  before  as  it  had  a  little  silver  in  it,  and 
fis  chalk  in  a  dish  to  an  open  window  where  the  sun  was 
lining  and  began  to  pour  the  acid  on  the  chalk.  He 
vas  soon  surprised  to  see  that  the  surface  of  the  mass 
:hanged  from  white  to  a  dark  violetish  red  where  the  sun 
hone  on  it,  but  not  where  the  edge  of  the  dish  shaded  it. 
ie  proceeded  to  investigate  the  cause  of  this  change,  using 
he  pasty  mixture  in  medicine  bottles  for  the  sake  of  con- 
lenience,  and  found  that  heat  would  not  produce  the 
olour.  Then  he  cut  out  words  or  even  entire  sentences 
(j1  paper,  like  stencil  plates,  attached  these  perforated 
apers  to  the  bottles  of  mixture  with  wax,  and  exposed 
iem  to  light.  The  words  were  then  clearly  visible  on  the 
intents  of  the  bottles,  to  the  wonderment  of  curious  on- 


73 


The  Development  of  Photography 

lookers.  By  shaking  the  bottle,  the  darkened  portions 
which  were  only  on  the  very  surface  of  the  sloPPy  ° 
pasty  mass,  became  mixed  up  with  the  bulk  an  J 

experiment  could  be  repeated.  He  investigated  he 
matter  and  found  that  it  was  the  silver  that  caused  the 
sensitiveness  to  light,  and  that  the  chalk  nught  be  replaced 
by  magnesia,  white  lead,  and  other  similar  substances.  He 
not  only  did  not  attempt  to  fix  the  photographs  that  lie 
produced,  but  was  particularly  careful  to  keep  the  mixture 
in  such  a  condition  that  it  could  easily  be  shaken  up  to 
get  rid  of  the  impression  and  so  be  ready  fo 

6XP\Ve  see  in  these  experiments  of  Schulze  the  true  | 
spirit  of  careful  observation  and  experimental  inquiry,  for 
during  the  previous  hundred  years  or  so  the  aims  an 
mysticism  of  alchemy  had  gradually  given  place  to  whs 
we  now  understand  as  scientific  methods.  It  is  easy  to  say 
that  whatever  Schulze  did  that  had  not  already  been  done 
was  not  worth  the  doing,  for  the  sensitiveness  of  sdver 
salts  to  light  had  been  known  long  before.  But  every 
little  step  helps  the  cause  forward,  and  the  emphatic 
demonstration  of  a  known  fact  may  be  even  more  lielpfu 
than  the  discovery  of  a  new  fact  or  a  new  application 

Silver  nitrate  at  this  time  had  been  known  for  hundreds 
of  years  both  in  the  solid  form  and  in  solution,  and  it  was 
used  in  surgery.  It  was  a  familiar  substance  among  those 
interested  in  such  matters,  and  it  is  not  surprising  ther«fH 
to  find  that  it  was  recommended  for  use  as  4  secre 
about  the  time  that  we  are  now  considering.  By  writi  g 
with  its  solution  on  paper  in  a  subdued  light  he  inscrip- 
tion  was  invisible,  but  on  exposure  to  hght  it  would 
gradually  grow  dark.  Not  a  very  practical  or  safe  secre 
fnk,  but  the  proposal  to  use  it  for  this  purpose  shows  that 
the  action  of  light  on  silver  compounds  was  becoming 
more  widely  known  and  that  there  was  a  tendency  to  apply 
it  to  useful  purposes. 


74 


The  Development  of  Photography 

Charles  William  Scheele,  a  Swede,  who  had  been  suc¬ 
cessful  in  isolating  chlorine  from  hydrochloric  acid  three 
years  before,  in  1777  investigated  the  change  that  light 
effects  in  chloride  of  silver.  Whether  or  not  the  alchemists 
knew  that  silver  chloride  became  darkened  by  exposure  to 
sunlight,  it  was  before  this  a  well  ascertained  and  recorded 
fact,  and  Scheele,  who  was  an  indefatigable  investigator, 
was  interested  in  the  action  of  light  in  general  and  especi¬ 
ally  in  its  relation  to  the  action  of  heat  upon  substances. 
He  prepared  a  quantity  of  silver  chloride,  dried  it,  and 
!  exposed  it  to  sunshine  for  two  weeks,  stirred  it  up  and 
|  exposed  it  again,  and  so  continued  until  he  obtained  a 
:  practically  black  powder.  This  he  treated  with  ammonia, 

!  which  dissolves  silver  chloride,  and  obtained  a  black 
•  .  .  ' 

:  insoluble  residue  which  he  identified  as  silver.  Hence  he 

i  concluded  that  the  action  of  light  consisted  in  the  separa¬ 
tion  of  metallic  silver  from  the  chloride  of  silver.  In  another 
1  experiment  he  exposed  to  the  light  some  chloride  of  silver 
in  water,  and  he  found  that  the  water  contained  the 
chlorine  that  the  light  had  separated  from  the  silver  salt. 
The  inferences  drawn  from  these  observations  we  know 
now  to  need  a  little  modification,  but  in  the  main  they  are 
correct.  Scheele  went  even  further  than  this,  for  he 
coated  some  paper  with  chloride  of  silver  and  found  that 
I  violet  light  acted  upon  it  to  darken  it  more  quickly  than 
|  any  other  rays  of  the  sun’s  spectrum. 

Experiments  of  the  kind  just  described  were  made  by 
other  investigators,  some  of  whom  carried  their  researches 
*  further  than  their  predecessors,  but  we  are  not  particularly 
concerned  with  them.  Our  present  aim  is  to  get  an  idea  as 
[to  how  matters  stood  at  the  end  of  the  eighteenth  century, 
that  we  may  better  understand  the  attitude  of  those  whose 
work  is  now  to  be  referred  to.  The  sensitiveness  of  various 
Isilver  compounds  to  light  was  then  well  known,  silver  salts 
had  been  spread  upon  paper  for  the  purposes  of  investiga¬ 
tion,  and  the  character  of  the  change  produced  by  light  had 

75 


The  Development  of  Photography 

been  discovered  by  an  examination  of  the  products  in  the 
case  of  silver  chloride  as  well  as  the  nature  of  the  active  light. 

We  may  not  think  much  of  a  suggestion  made  by  Lor 
Brougham  in  1795  when  he  was  a  youth  of  seventeen,  to 
rub  silver  nitrate  on  ivory  and  on  the  surface  to  r^cei^e 
and  so  render  “permanent”  the  picture  produced  by  ie 
camera.  This  suggestion  was  deleted  from  a  communica¬ 
tion  that  Brougham  sent  to  the  Royal  Society,  and  he  does 
not  appear  to  have  considered  that  it  was  worth  taking 

further  trouble  about.  .  .  ...  . 

Thomas  Wedgwood,  the  fourth  son  of  Josiah  Wedg¬ 
wood  the  renowned  potter,  was  born  in  1771  and  suffered 
all  his  short  life  (he  died  at  thirty-four  years  of  age)  from 
ill-health  which  gradually  unfitted  him  for  any  work  except 
travelling  in  the  endeavour  to  mitigate  his  sufferings.  He 
had  a  distinct  taste  for  scientific  pursuits,  and  was  interested 
especially  in  the  effects  produced  by  light  and  heat.  He 
worked  with  a  solar  microscope,  that  is  an  apparatus  that 
utilises  the  direct  rays  of  the  sun  to  give  on  a  screen  an  en¬ 
larged  image  of  suitable  small  objects,  and  he  became 
anxious  to  make  “permanent”  the  images  so  produced. 
The  word  permanent  here  does  not  mean  that  the  picture 
should  of  necessity  last  for  long,  but  only  that  the  picture 
should  be  so  “fixed”  on  the  screen  that  it  should  remain 
after  the  apparatus  producing  it  was  removed.  For  this 
purpose  he  used  silver  salts,  especially  the  nitrate,  and 
probably  worked  on  and  off  as  circumstances  permitted 
for  some  years  in  his  endeavours  to  get  satisfactory  results. 

Sir  Humphry  Davy  began  to  study  chemistry  in  1798, 
in  the  following  year  he  began  an  important  series  o 
experiments  011  the  inhalation  of  nitrous  oxide,  and  in 
1801  he  left  an  appointment  that  he  held  at  Bristol  to 
come  to  the  newly  established  Royal  Institution  in  London. 
When  Davy  came  to  London  he  was  a  young  man  twenty- 
three  years  of  age  and  Wedgwood  was  twenty-nine. 
Wedgwood  showed  Davy  his  results  either  in  this  or  the 

76 


A  Sunset  Sky 


The  Development  of  Photography 

following  year,  perhaps  seeking  the  advice  of  the  young 
chemist  with  regard  to  some  means  for  preventing  the 
light  from  obliterating  his  photographs  by  blackening  all 
over  the  paper  on  which  he  had  produced  them.  All  that 
is  really  known  is  that  in  1802  there  was  published  in  the 
1  Journal  of  the  Royal  Institution  a  communication  with 
I  the  title  **  On  an  account  of  a  method  of  copying  paintings 
upon  glass,  and  of  making  profiles,  by  the  agency  of  light 
upon  nitrate  of  silver.  Invented  by  T.  Wedgwood,  Esqr. 

!  With  observations  by  H.  Davy."  Paper  or  leather  was 
moistened  with  a  solution  of  nitrate  of  silver  and  the  image 
allowed  to  fall  upon  it.  The  colour  of  the  impression 
produced  could  not  be  washed  out  with  soap  and  water, 
but  all  attempts  by  repeated  washing  or  varnishing  to 
render  the  uncoloured  portion  of  the  paper  indifferent  to 
light,  to  u  fix  ”  the  print  as  we  should  now  say,  were 
unsuccessful.  The  hyposulphites,  the  ordinary  fixing  salt 
of  to-day,  had  been  discovered  three  years  before,  but 
it  was  only  a  laboratory  curiosity  and  perhaps  it  had  not 
come  under  Davy’s  observation.  Leaves,  insects’  wings, 
and  ordinary  printing  press  prints  could  be  copied  slowly 
[by  putting  the  sensitive  paper  in  contact  with  the  original 
and  exposing  to  light,  but  a  camera  image  was  too  faint  to 
produce  a  result.  Davy  says  that  to  secure  the  camera 
jimage  was  the  first  object  of  Mr.  Wedgwood,  but  that 
ina  this  all  his  numerous  experiments  were  unsuccessful. 
Davy  tried  silver  chloride  and  found  it  more  sensitive  than 
he  nitrate.  He  refers  to  some  possible  future  experi- 
nents  with  regard  to  “  destroying "  the  sensitive  com¬ 
pound  not  acted  on  by  light  (fixing),  and  adds  that  this  is 
ill  that  is  needed  “to  render  the  process  as  useful  as  it  is 
plegant."  These  further  experiments  do  not  seem  to  have 
peen  made.  Three  years  after  this  Wedgwood  died,  and 
pavy,  who  was  busy  with  the  duties  of  his  position  and  his 
hemical  investigations,  probably  gave  no  further  thought 
P  the  matter.  In  1829  Davy  died. 

77 


1 


The  Development  of  Photography 

The  next  two  men  that  we  have  to  refer  to,  worked 
at  the  perfection  or  discovery  of  a  photographic  process 
with  an  unprecedented  perseverance.  They  are  the  firs 
men  who  really  devoted  themselves  to  the  matter.  Josep 
Nicephore  Niepce  was  born  at  Chalons-sur-Saone  in  7  5. 
six  years  before  Thomas  Wedgwood  was  born,  educated  I 

for  the  Church,  but  at  the  time  of  the  French  revolution 
became  a  soldier.  After  this  he  settled  down  at  his  birth¬ 
place  and  indulged  his  scientific  tastes.  In  1815,  or about 
then  he  began  his  photographic  experiments  with  the 
idea  of  finding  if  possible  a  method  of  automatically  copy¬ 
ing  designs  upon  lithographic  stones,  so  as  to  save  the 
tedious  work  of  the  draughtsman.  Lithography  was 
comparatively  new  art  at  that  time,  and  was  just  beginm 
toT  appreciated  in  France.  From  the  stone,  he  passed 
eventually  to  tin,  pewter,  silver,  and  even  glass, 
sensitive  substance  that  he  was  most  successful  with  was 
bitumen  or  asphalte,  which  he  dissolved  to  form  a  varnish 
and  then  applied  so  as  to  obtain  a  film  on  the  surfac 
to  be  treated.  This  film  of  varnish  when  exposed  to  lig 
would  show  no  sign  of  change,  but  where  the  light  acted 
it  would  become  less  soluble,  so  that  on  applying  a  weak 
solvent,  such  as  a  mixture  of  oil  of  lavender  and  petroleum 
oil,  the  parts  that  were  not  changed  by  the  action  of  | 
the  light  would  dissolve  more  readily  than  the  othei  parts, 
and  with  due  care  a  good  picture  would  be  °b?a'ne°’ 
the  varnish  left  on  the  plate  corresponding  to  the  light  or 
white  parts  of  the  subject.  Niepce  succeeded  in  getting 
pictures  in  the  camera  with  six  or  eight  hours  exposure, 
and  he  could  copy  a  transparent  print  laid  direct  upon 
the  plate  and  exposed  to  light  in  about  two  hours.  °ome! 
of  these  pictures  on  metal  he  put  into  acid  that  dissolve  j! 
or  etched  the  metal  where  it  was  not  protected  by  thej 
varnish,  and  from  these  he  got  prints  by  the  ordinary, 
method  of  the  printing  press.  The  pictures  produced 
on  silver  showed  the  reddish  varnish  in  the  parts  corre-| 

78 


The  Development  of  Photography 

sponding  to  the  bright  parts  of  the  object,  and  the  white 
metal  corresponding  to  the  dark  parts  or  shadows.  He 
tried  to  darken  the  metal  where  it  was  exposed  to  make 
the  appearance  more  natural,  and  the  substance  that  he 
was  the  most  successful  with  for  this  purpose  was  iodine, 
which  does  darken  silver  very  effectually.  The  varnish 
could  then  be  dissolved  away,  and  now  the  white  silver 
would  represent  the  lights  and  the  darkened  silver  the 
dark  parts  of  the  object  or  print. 

Thus  Niepce  was  eminently  successful  from  an  experi¬ 
mental  point  of  view,  for  he  not  only  got  his  pictures, 
even  those  produced  in  the  camera,  but  he  succeeded 
in  fixing  them,  using  the  word  “  fix  ”  in  its  present-day 
sense,  and  those  of  his  pictures  that  have  been  taken  care 
pf  show  little  or  no  deterioration.  Moreover  he  produced 
nvisible  images  which  he  developed. 

Louis  Jacques  Mand6  Daguerre  was  twenty- two  years 
he  junior  of  Ni6pce  and  a  very  different  sort  of  man. 
de  was  born  near  Paris,  and  was  perhaps  the  most 
celebrated  scene-painter  who  has  ever  lived.  He  was  not 
>nly  successful  as  a  man  of  business,  but  he  was  ingenious 
nd  original  in  his  methods.  He  used  a  camera  obscura 
3  help  him  in  his  painting,  and  about  the  year  1824 
/as  moved  with  a  desire  to  try  to  “fix  ”  the  camera  image. 
Ve  do  not  know  what  gave  rise  to  this  desire,  whether 
e  hoped  that  it  would  help  him  in  his  drawing,  or 
'hether,  as  seems  more  likely,  he  thought  that  if  successful 
ie  production  of  pictures  in  this  way  would  excite  the 
iterest  of  the  public  and  be  profitable  to  him.  He  seems 
>  have  been  working  with  phosphorescent  substances, 
tat  is  substances  which  when  illuminated  continue  to 
fine  after  the  light  that  illuminates  them  is  withdrawn, 
erhaps  he  hoped  to  get  self-luminous  pictures  in  this 
ay,  for  we  know  that  he  was  original  and  successful 
the  manner  in  which  he  illuminated  his  dioramas, 
larles  Chevalier,  who  was  a  maker  of  optical  instruments 

79 


The  Development  of  Photography 

and  had  a  shop  in  Paris,  used  to  supply  Daguerre  with 
apparatus,  and  he  knew  the  direction  in  which  he  was 
working.  In  1826,  either  Niepce  or  his  father  went  to 
Chevalier’s  to  see  about  getting  a  camera,  and  in  this 
way  Chevalier  came  to  know  of  Niepce’s  success.  He 
told  Daguerre  of  this,  and  Daguerre  immediately  wrote 
to  Niepce  on  the  subject.  Niepce  was  reticent  and  j 
cautious,  but  Daguerre  was  genial  and  persevering,  and 
the  result  was  that  in  1829  the  two  men  entered  formal  y 
into  partnership.  According  to  the  agreement,  Niepce  told 
Daguerre  how  he  had  succeeded,  and  in  return  Daguerre 
contributed  some  detail  with  regard  to  an  improvement 
in  his  camera.  Daguerre,  the  pushful  and  younger  man, 
seems  to  have  had  the  best  of  the  bargain,  but  leP^ 
had  been  disappointed  that  his  results,  which  he  brought  , 
also  to  this  country,  awakened  practically  no  interest, 
even  among  scientific  men.  Ni6pce  was  now  sixty-four 
years  old,  and  four  years  after  this  he  died. 

Niepce  used  iodine  to  blacken  the  silver  plate,  as 
already  stated,  after  he  had  got  his  photograph,  and  it 
was  doubtless  this  that  led  Daguerre  to  work  with  silver 
darkened  on  its  surface  by  iodine  fumes.  But  Daguerre  | 
used  this  darkened  silver  surface  as  the  sensitive  surface,  • 
which  Niepce  appears  never  to  have  done.  By  1837, 
Daguerre  had  by  sheer  perseverance,  so  great  that  his  j 
wife  had  doubts  as  to  his  sanity,  brought  the  Daguerreo- 
type  process  to  a  practical  form.  He  and  Niepces  son,  ^ 
Isidore,  endeavoured  to  put  the  process  on  a  commercial; 
footing,  but  the  incredulity  and  indifference  of  others 
they  could  not  overcome.  Probably  as  a  last  resource  J 
Daguerre  went  to  Arago,  the  most  celebrated  scientific 
man  in  France,  and  he  saw  enough  of  the  possibilities; 
of  the  process  to  become  enthusiastically  interested  in 
it.  Through  Arago’s  influence  Daguerre  and  the  younger! 
Niepce  were  given  substantial  pensions  by  the  b  renc  j 
Government  on  condition  that  they  published  the  detai  9 

80 


The  Development  of  Photography 


of  the  process,  and  this  was  done  in  1839.  The  process 
consisted  in  exposing  the  polished  surface  of  a  silvered 
copper  plate  to  the  fumes  of  iodine  until  it  was  coated 
with  a  compound  of  silver  and  iodine.  The  plate  was 
exposed  in  the  camera,  and  then  developed  by  putting 
it  over  a  dish  of  gently  warmed  mercury.  The  mercury 
vapour  deposited  on  those  parts  where  the  light  had 
acted,  but  not  on  the  other  parts.  Where  the  light  had 
acted  to  a  small  extent,  there  was  a  correspondingly  small 
deposition  of  mercury,  so  that  the  lights  and  shades  were 
well  reproduced.  The  silver  iodide  was  then  dissolved 
away  by  hyposulphite  of  soda,  just  as  negatives  and  silver 
prints  are  fixed  at  the  present  day. 

Daguerreotype  was  the  first  method  of  photography 
available  for  practical  purposes.  As  soon  as  the  neces¬ 
sary  details  could  be  obtained  a  great  many  persons  took 
up  the  process.  The  plates  were  eventually  made  more 
sensitive  by  the  use  of  bromine  in  addition  to  the  iodine, 
ind  other  improvements  were  effected ;  and  Petzval  de¬ 
mised  his  portrait  lens,  which  still  further  reduced  the  very 
ong  exposures  at  first  necessary.  Daguerreotypists  gradu¬ 
ally  increased  in  number  in  practically  all  parts  of  the 
civilised  world,  and  photography  became  a  “  profession.” 
>0  far  as  the  general  public  were  concerned  the  process 
'ad  no  rival  until  after  1851,  the  year  in  which  Daguerre 
led  and  the  collodion  process  first  saw  the  light.  A  few 
ears  after  this  the  Daguerreotype  process  became  practi- 
ally  obsolete. 

The  announcement  of  Daguerre's  success  in  1839, 
>ming  with  the  stamp  of  the  authority  of  the  French 
overnment,  at  once  attracted  attention.  Indeed  before 
was  announced  some  preliminary  information  was  circu- 
ted,  and  this  urged  into  increased  activity  others  who 
sre  seeking  to  make  photography  practical.  W.  H.  Fox 
[ilbot,  who  had  been  working  on  similar  lines  to  Wedg¬ 
ed  and  Davy,  sent  two  communications  to  the  Royal 

81  F 


The  Development  of  Photography 


Society  early  in  1839  showing  that  he  had  obtained  a 
much  greater  sensitiveness  than  the  earlier  workers  an 
had  succeeded  in  “fixing"  the  result.  He  used  silver 
chloride  for  his  sensitive  substance,  impregnated  paper  with 
it  for  use,  and  fixed  either  with  a  weak  solution  of  potas¬ 
sium  iodide  or  a  strong  solution  of  common  salt.  Three 
weeks  after  this  Sir  John  Herschel,  who  had  heard  seven 
weeks  before  that  Daguerre  had  a  successful  process  ready 
to  be  made  public,  accepted  the  statement  “as  an  enigma 
to  be  solved,"  and  finding  silver  chloride  on  paper  lack¬ 
ing  in  sensitiveness  used  nitrate  of  silver.  For  removing 
the  unaltered  silver  compound  after  the  exposure  in  order 
to  fix  the  photograph,  he  used  a  hyposulphite,  which  he 
had  previously  recommended  for  this  purpose.  He  mace 
not  only  what  we  now  call  negatives,  but  also  “second 
transfers,”  as  he  called  them,  or  what  we  now  under¬ 
stand  as  prints  or  positives.  He  showed  twenty-three 
photographs,  “  one,  a  sketch  of  his  telescope  at  Slough 
fixed  from  its  image  in  a  lens;  and  the  rest  copies  of 
engravings  and  drawings."  There  was  one  other  worker 
of  note  who  brought  forward  his  results  at  this  time, 
namely  the  Rev.  J.  B.  Reade,  a  scientific  man  of  a  retiring 
disposition,  who  appears  to  have  been  the  only  one  that 
really  sought  to  follow  up  the  experiments  of  Wedgwood. 
Early  in  1837  he  was  anxious  to  save  the  cost  of  an 
artist  for  drawing  the  magnified  pictures  that  he  pro¬ 
jected  on  a  screen  by  means  of  his  microscope,  using 
either  sunshine  or  the  oxyhydrogen  light,  and  so  endea¬ 
voured  to  get  photographic  impressions.  As  Wedgwood 
and  Davy  had  found  that  leather  gave  a  more  sensitive 
surface  than  paper  when  impregnated  with  the  silver  salt, 
he  used  light  coloured  leather  gloves  until  the  supply 
failed.  Then  he  said  “  1  will  tan  paper."  For  this  pur¬ 
pose  he  applied  an  infusion  of  galls  to  the  paper  in 
addition  to  the  silver  salt,  and  so  obtained  a  greatly  en¬ 
hanced  sensitiveness.  He  obtained  a  photograph  o  a 

82 


The  Development  of  Photography 

flea  by  means  of  his  microscope  in  less  than  five  minutes 
and  sections  of  wood  were  photographed  with  an  exposure 
of  from  eigh  to  ten  minutes.  The  sensitive  paper  so 
ared,  Could  be  used  m  the  camera.  Reade  used 
silver?althyP°SU  Phlte  ^  dissolving  away  the  unaltered 

Two  years  later,  Fox  Talbot  effected  a  very  great 

thaTa  V"  ahe  ,Se"sltlveness  of  his  PaPer,  making  it  more 
undred  times  as  sensitive  as  any  known  before 

e  says.  He  had  accidentally  discovered  that  it  was 
possib  e  to  produce  with  silver  compounds  a  latent  or 
mvis.We  impression  that  could  afterwards  be  developed. 
Both  Daguerre  and  Niepce  developed  their  photographs 
rom  invisible  images,  but  this  kind  of  development  appears 
to  be  radically  different,  as  we  shall  see  in  detail  when 

Fn v "t'o u  t°  c°nsjder  *he  subject.  In  this  process  of 
Fox  Talbot  s,  which  was  called  "calotype,”  paper  was  im- 

oief  w ith  W‘  Sll!,er  i0,dide’  and  shortly  before  use  washed 
icid  *  m‘Xed  SO'Ution  °f  silver  nitrate  and  gallic 
but  exposure  in  the  camera  no  change  was  visible, 

nore  IT  S'  Paf>er  WaS  "ashed  OTer  with 

”  6  °f utbe  gall,c  acid  and  silver  solution  and  gently 
varmed  before  the  fire  the  image  gradually  appeared  and 

'ieiT  1  "Se  “fckness-  With  an  ordinary  slow 

ew  lens  one  minutes  exposure  would  suffice  for  a 

rhite"1^  *n  sunshine,  and  with  a  rapid  portrait  lens  a 
te  bust  m  sunshine  needed  only  one  second  to  pro- 
uce  an  impression  that  would  satisfactorily  develope. 

,e  ^mceSe  neg!f‘VeS  F°X  Talbot  made  Prints  in  much 
ie  same  way  as  they  are  made  at  the  present  time. 

Var  ,l  Jw'11  be  observed  ‘hat  the  sensitive  silver  com- 
yunds  used  were  either  on  a  metal  plate  or  on  and 

°  ,leSS  !n  the  substance  of  a  sheet  of  paper.  In 

l  ed’  tiPCe  ?e  Saint  V‘ctor,  nephew  of  the  first  Niepce, 

-  a  him  of  albumen  supported  on  glass  to  hold  the 
ive  compound,  that  is  a  transparent  film.  In  i8ci 

83 


The  Development  of  Photography 

Frederick  Scott  Archer  published  a  proces s  m 

lodion  was  used  instead  of  albumen.  But  this  is  a  proces 

that  still  survives,  and  therefore  must  receive  more  deta.le 

atteWe0haveethus'traced  photography  with  silver  salts  from 

caUv  refer  But  if  all  the  historical  facts  mentioned  in 
thisyvolutne  were  brought  together,  they  would  not  con- 
stitute°  anything  like  a  complete  history  of  the  subject. 
T  ere  were  other  workers  besides  those  mentioned,  before 
[0“  Zd  since  then  there  have  been  very  many  indeed. 
They  have  been  of  all  kinds,  from  the  most  learned  scien¬ 
tific  ^en  to  those  who  knew  nothing  of  scientific  metho 
and  ha  dly  more  of  what  we  understand  as  scientific 
facts  They  have  been  from  the  wealthiest  to  the  poorest 
Sonm  have  worked  for  practical  results  only,  some  for 
personal  profit,  and  some  rather  to  elucidate  a  theory '  or 
to  discover  the  exact  nature  of  the  changes  tha 
Place  Photography  is  so  varied  in  its  methods  a 
applications  that  it  may  be  regarded  from  many  poin 

°f  'ph™re  is  only  one  other  word  appropriate  to  this! 
chanter  The  general  reader  may  find  some  description! 
rf processes  not  very  clear  to  him.  To  have  burdenec 
the  historical  details  with  explanations  of  the  changes  re 
ferred  to  would  have  led  to  confusion,  but  we  hope  tha 
any  such  difficulties  will  be  found  to  be  removed  in  , 
subsequent  chapters. 


84 


CHAPTER  V 

PHOTOGRAPHY  BEFORE  GELATINE 


In  following  the  development  of  photography  from  its 
beginnings  it  is  clear  that  the  compounds  of  silver  have 


always  occupied  a  very  important  position.  But  innumer¬ 
able  substances  are  changed  by  the  action  of  light,  and 
if  silver  and  its  compounds  had  never  existed  photography 
would  still  be  not  only  possible  but  practicable.  No  two 
substances  that  we  can  distinguish  from  each  other  are 
alike,  for  if  they  were  they  would  be  only  different 
portions  of  the  same  substance,  nor  are  the  changes 
that  they  undergo  exactly  the  same.  When  therefore 
we  have  a  large  number  to  select  from,  the  problem 
is  to  choose  those  that  offer  the  most  substantial 
.advantages. 

The  changes  that  light  produces  in  silver  compounds 
certainly  were  and  probably  still  continue  to  be  the  most 
obvious  of  all  such  changes.  Under  suitable  conditions 
diver  chloride  darkens  almost  to  blackness  by  an  exposure 
°  daylight  that  is  not  very  prolonged.  This  fact  created 
1  prejudice  in  favour  of  silver  compounds,  and  it  was 
his  that  led  to  the  discovery  by  Fox  Talbot  that  a 
■hort  exposure  to  light,  so  short  as  to  give  no  visible 
esult,  produced  an  instability  in  the  parts  exposed  that 
nabled  a  suitable  substance  to  produce  the  blackness, 
p  application  after  the  light  had  been  withdrawn. 
;^nd  it  is  in  this  that  the  particular  value  of  silver  com¬ 
pounds  consists. 

What  concerns  us  especially  now  is  that  by  this  course 
'f  events  the  brighter  the  object  is,  the  denser  is  the 


85 


Photography  before  Gelatine 

deposit  produced  by  its  action  on  the  sensitive  surface  in 
the  camera,  and  if  this  deposit  is  dark,  the  bright  parts 
of  the  object  are  represented  by  dark  deposits  in  the 
image.  The  bright  sky  is  represented  by  blackness  on  j 
the  plate,  and  the  dark  foliage  leads  to  the  production 
of  very  little  deposit,  so  that  we  get  a  total  reversal  o 
light  and  shade.  That  brightness  should  produce  darkness 
seemed  to  tickle  the  fancy  of  Schulze,  who  started  out, 
as  Daguerre  appears  to  have  done,  by  the  endeavour  to 
make  or  use  a  preparation  that  would  shine  more  brightly 
in  proportion  to  the  brightness  of  the  light  that  fell  upon 
it,  as  we  now  say  would  phosphoresce.  This  that  seemed 
to  be  a  grave  disadvantage  to  some  of  the  earlier  workers, 
has  proved  to  be  of  the  greatest  service,  because  if  this 
reversal  of  light  and  shade  can  be  obtained  once  it  can 
be  obtained  a  second  time.  If  the  first  photograph  is 
used  as  a  shield  to  a  piece  of  sensitive  material  placed 
close  against  it  while  the  two  are  exposed  to  light,  the 
black  sky  protects  the  sensitive  surface  beneath  it 
and  then  there  is  little  or  no  action,  while  the  thin 
deposit  in  the  shadows  allows  the  light  to  pass  more  j 
freely  and  produce  the  darkness  that  corresponds  with 
the  original  object.  Sir  John  Herschel  called  the  first 
photograph  a  “reverse  transfer”  or  “first  transfer/  and 
the  second  “a  second  transfer  or  re-reversed  picture.’  j 
Shortly  afterwards,  “to  avoid  circumlocution,”  he  called 
the  first  a  “  negative  ”  and  the  second  a  “  positive,  and 
although  these  words  do  not  etymologically  convey  the  ! 
meaning  attached  to  them,  they  have  served  their  purpose 
in  photographic  nomenclature  for  seventy  years,  and  doubt¬ 
less  will  continue  to  do  so.  The  great  advantage  of  this 
method  of  work  is  that  a  negative  will  give  any  number 
of  positives  or  “  prints  ”  without  any  further  recourse  to 

the  original  object. 

But  this  method  of  work  has  another  advantage,  in 
addition  to  the  facility  that  it  offers  for  multiplying  the 

86 


Photography  before  Gelatine 

photographs,  that  is  not  often  appreciated.  Any  photo¬ 
graph  that  is  obtained  directly  in  the  camera  as  a  Da¬ 
guerreotype  is,  is  “  reversed  "  in  quite  a  different  sense  from 
the  reversal  just  referred  to.  It  is  optically  or  laterally 
reversed.  You  have  only  to  look  at  yourself  in  a  looking- 
glass  to  see  an  example  of  lateral  inversion.  If  you  raise 
your  right  hand,  the  image  that  you  see  in  the  glass  raises 
his  hand  that  is  on  the  same  side,  but  as  the  image  faces 
you  it  is  his  left  hand.  Thus  we  have  a  lateral  or  side¬ 
ways  inversion,  and  all  photographs  taken  direct  suffer  in 
this  way.  A  negative  is  laterally  reversed,  but  the  positive 
or  print  made  from  it,  as  the  two  are  face  to  face  in 
making  the  print,  is  reversed  again  and  so  corrected.  A 
negative  looked  at  through  from  the  back  shows  the  image 
without  lateral  inversion. 

Anyone  who  is  not  familiar  with  this  phenomenon  of 
lateral  inversion  may  find  another  illustration  help  to  make 
its  character  more  clear.  Take  a  piece  of  tracing  paper, 
or  the  equivalent,  and  draw  on  it  in  ink  some  design  with 
different  sides  to  it,  such  as  the  profile  of  a  person's  face 
or  the  letter  F.  Looking  at  the  side  of  the  paper  that 
you  have  written  on  the  writing  is  not  reversed,  but  by 
turning  the  paper  over  and  looking  at  it  from  the  back  the 
design  is  laterally  reversed,  the  face  or  the  horizontal 
strokes  of  the  letter  point  in  the  opposite  direction.  Now 
hold  up  the  paper  with  its  under  side  to  a  looking-glass 
and  look  at  the  back  of  the  paper  as  seen  reflected  in  the 
glass,  and  the  reversed  image  will  be  found  to  be  re¬ 
reversed  by  the  reflection  and  so  to  appear  in  its  correct 
position. 

This  lateral  inversion  would  of  course  be  intolerable 
|  in  a  portrait,  as  it  would  represent  the  person  as  if  he  were 
|  left-handed,  any  mark  on  one  side  of  his  face  would 
appear  on  the  other  side,  his  coat  would  button  over  the 
j  wrong  way,  his  watch  and  his  handkerchief  would  be  in 
I  pockets  on  his  right  hand  side,  his  hair  would  be  parted  on 

87 


Photography  before  Gelatine 

the  right  instead  of  the  left,  and  everything  would  be 
wrong.  This  difficulty  used  to  be  overcome  in  making 
Daguerreotypes  by  putting  a  mirror  in  front  of  the  lens 
and  at  an  angle  with  it,  and  photographing  the  reflected 
image  instead  of  the  person  direct.  A  reflecting  prism 
was  sometimes  used,  but  the  principle  remains  exactly  the 
same.  Mirrors  and  prisms  are  in  constant  use  at  the 
present  day  in  order  to  correct  lateral  inversion  in  those 
processes  that  otherwise  would  cause  the  final  pictures  to 
suffer  from  it. 

Reverting  to  the  method  of  making  negatives  for  the 
purpose  of  getting  prints  from  them,  we  find  that  the  use 
of  paper  as  a  support  for  the  sensitive  silver  compound 
had  many  disadvantages.  The  water-mark,  if  present,  and 
other  variations  in  its  thickness,  the  impurities  and 
foreign  matters  that  might  be  present,  the  possible  varia¬ 
tions  in  the  sizing  of  it  (that  is  the  addition  of  substances  to 
render  it  less  absorbent  as  blotting-paper  is),  all  tended  to 
introduce  irregularities.  Again,  paper  is  not  very  trans¬ 
parent  and  even  if  impregnated  with  waxy  substances,  as 
was  often  done,  it  still  left  much  to  be  desired  in  this 
particular.  Glass  had  been  used  by  Niepce,  though  not 
apparently  for  the  sake  of  getting  what  we  understand 
as  a  negative.  Sir  John  Herschel  in  1840  used  glass,  and 
got  his  silver  salt  upon  it  by  putting  it  at  the  bottom  of  a 
vessel  that  contained  the  silver  salt  suspended  in  water, 
and  allowing  the  silver  salt  to  settle  down  upon  it.  By 
carefully  removing  the  plate  and  drying  it,  the  silver  salt 
adhered  sufficiently  to  permit  of  the  necessary  operations, 
if  due  care  was  taken.  He  found  that  it  was  possible  to 
get  prints  from  photographs  produced  on  such  plates, 
though  he  did  not  make  them  especially  for  use  as 
negatives,  but  the  inconvenience  and  risk  in  both  the 
preparation  and  use  of  them  would  preclude  them  from 
being  generally  employed. 

Others  endeavoured  to  use  glass  plates  instead  of  paper, 

88 


An  Old  Doorway  of  Marston  Trussell  Church 

Uncovered  a  few  years  ago. 


C.J. 


Photography  before  Gelatine 

but  it  was  not  until  the  nephew  of  Niepce,  Niepce  de  Saint 
Victor,  in  1848,  worked  out  a  method  of  using  a  film  of 
albumen  to  hold  the  sensitive  compound  on  to  the  glass, 
that  the  production  of  negatives  on  glass  became  a  practi¬ 
cally  useful  method.  Other  substances  were  tried  as  well 
as  albumen,  but  without  much  success  until  Frederick 
Scott  Archer  published  in  1851  a  method  that  he  had 
elaboiated  of  using,  collodion  for  this  purpose.  The 
collodion  process  is  still  in  use  for  certain  purposes  so  that 
it  demands  a  more  detailed  consideration. 

Photography  as  a  practical  art  dates  from  the  intro¬ 
duction  of  the  Daguerreotype  and  the  paper  processes  of 
Fox  Talbot  in  1839.  There  have  been  only  two  other 
epochs  of  first  importance,  namely  the  collodion  process  in 
1851  and  the  gelatine  process  about  twenty  years  after. 
The  introduction  of  these  newer  processes  did  not  mean  a 
sudden  revolution  in  photography  as  an  art,  for  an  innova¬ 
tion  was  always  regarded  with  a  measure  of  scepticism. 
It  was  necessary  to  convince  those  who  were  actively 
engaged  in  the  pursuit  of  the  advantages  of  the  new 
process,  then  the  workers  had  by  practice  to  become 
skilled  in  the  working  of  it  and  their  apparatus  had  to  be 
adapted  to  it.  So  we  find  that  Daguerreotypes  were  made 
commercially  for  five  or  six  years  after  collodion  was  used, 
but  the  facilities  and  economy  of  the  collodion  process 
finally  drove  the  Daguerreotype  process  into  the  obsolete 
methods  of  the  past.  For  about  five-and-twenty  years 
collodion  held  its  own  without  a  rival.  It  was  collodion 
that  popularised  photography,  for  a  Daguerreotype  was  a 
single  and  costly  picture,  while  by  the  use  of  collodion 
those  who  enjoyed  but  slender  incomes  could  afford  to  have 
their  photographs  taken  now  and  then,  and  if  they  wished 
they  could  have  copies  for  distribution. 

There  were  two  collodion  processes,  the  negative  and 
the  positive,  the  latter  giving  a  single  picture  which  was 
always  put  into  a  frame  of  some  sort  after  the  style  of  a 

89 


Photography  before  Gelatine 

Daguerreotype.  In  the  negative  process,  the  photographer 
made  a  negative,  and  from  it  printed  the  desired  number 
of  copies  on  paper  which  were  finally  mounted  on  cards. 

It  is,  practically  speaking,  the  same  process,  ™hetl|ier  " 
is  wished  to  make  a  negative  or  a  positive,  and  only  th 
details  are  varied  according  to  the  end  in  view. 

Collodion  is  a  solution  of  a  kind  of  gun-cotton  in  a 
mixture  of  alcohol  and  ether,  and  when  the  solution  is 
poured  upon  any  surface  the  solvents  evaporate  and  leave 
a  continuous  film  or  varnish  of  the  gun-cotton  Cotton 
when  pure  is  a  definite  substance  called  cellulose,  and 
cotton  wool  and  good  white  blotting-paper,  such  as  is  used 
for  filtering  purposes  in  chemical  laboratories,  are  nearly 
pure  cellulose.  When  cellulose  is  put  into  a  mixture  o 
nitric  and  sulphuric  acids,  nitrates  of  cellulose  are  pro¬ 
duced.  If  the  action  is  pushed  on  as  far  as  possible  a 
nitrate  is  produced  which  if  dried  and  set  on  hre  burns 
with  a  sudden  burst  of  flame.  This  is  the  gun-cotton  that 
is  used  in  warfare  and  for  blasting  in  mines  and  quarries. 

If  the  action  of  the  acids  is  moderated  by  adding  a  little 
water  or  shortening  the  time  that  the  cotton  is  subjected 
to  their  action,  a  less  explosive  compound  is  produced 
which  is  called  “  soluble  gun-cotton  "  or  “  pyroxyhne,  and 
it  is  this  substance  that  is  dissolved  in  a  mixture  of  alcoho 
and  ether  to  prepare  collodion.  There  is  no  possibility  o 
accident  from  the  combustibility  of  the  basis  of  collodion 
when  employed  in  the  usual  manner,  for  however  com¬ 
bustible  a  material  may  be,  it  cannot  be  set  lire  to  when  it 
is  spread  as  a  thin  varnish-like  film  upon  glass  or  any| 
similar  support.  The  old  experiment  of  wrapping  a  piece 
of  paper  tightly  round  a  poker  and  then  holding  it  in  a 
flame,  shows  that  close  contact  with  a  considerable  massj 
of  material  keeps  a  layer  of  combustible  material  coo 
even  when  a  source  of  heat  is  applied  to  it. 

Although  the  collodion  film  plays  the  same  part  as  the 
paper  did  in  the  older  process  in  being  the  vehicle  that  is 

90 


Photography  before  Gelatine 

impregnated  with  the  sensitive  compound,  it  is  not  con¬ 
venient  to  prepare  the  film  first  and  then  treat  it  as  the 
paper  was  treated.  The  sensitive  compound,  silver  iodide 
in  this  case,  is  obtained  by  bringing  together  two  sub¬ 
stances  that  produce  it  by  their  action  upon  each  other, 
and  it  is  convenient  to  put  one  of  these  into  the  liquid 
collodion  before  the  glass  is  coated  with  it.  As  the  change 
produced  by  light  during  the  exposure  in  the  camera  is 
exceedingly  minute  and  invisible,  serving  only  to  render 
the  silver  compound  amenable  to  the  developer,  great  care 
has  to  be  taken  that  the  silver  iodide  is  produced  and  kept 
in  the  dark,  so  that  no  light  reaches  it  at  any  time  what¬ 
ever  until  it  receives  in  the  camera  the  image  of  the  object 
to  be  photographed. 

The  collodion  before  use,  therefore,  has  added  to  it  a 
solution  of  an  iodide  that  is  soluble  in  the  collodion  sol- 
vents,  generally  iodide  of  potassium,  of  ammonium,  or  of 
cadmium.  It  is  then  ready  for  application  to  the  glass 
plate.  This  must  be  scrupulously  clean,  for  any  ordinary 
dirty  matter  would  be  very  likely  to  interfere  with  the 
action  of  the  developer,  either  facilitating  it  and  so  causing 
a  deposit  on  development  where  it  should  not  be,  or 
hindering  it  and  so  leading  to  the  production  of  parts  that 
are  unduly  transparent.  For  a  similar  reason  the  collodion 
cannot  be  applied  with  a  brush  as  is  often  customary  with 
varnishes,  for  it  would  be  impossible  to  keep  the  brush 
clean.  The  use  of  a  brush  too  would  result  in  an  uneven 
layer,  and  this  clearly  would  be  detrimental.  The  pre¬ 
pared  collodion  is  therefore  poured  on  to  the  glass  plate 
while  this  is  held  in  a  horizontal  position,  and  by  a  dex¬ 
terous  sloping  of  the  plate  the  liquid  is  caused  to  flow  all 
over  it  and  the  excess  is  allowed  to  drain  off  at  one  corner. 
A  small  plate  is  held  in  the  hand,  while  a  plate  that  is 
too  large  and  heavy  for  manipulation  in  this  way,  and 
a  piece  of  plate  glass  two  or  three  feet  square  is  of  a 
considerable  weight,  is  allowed  to  rest  near  its  centre 

9i 


Photography  before  Gelatine 

on  a  well  padded  and  suitably  supported  round  cushion 
two  or  three  inches  in  diameter.  On  such  a  contrivance 
the  workman  can  manipulate  large  and  heavy  plates  wit 

ease. 

When  the  film  of  collodion  has  set,  but  before  it  has 
dried,  the  plate  is  placed  in  a  solution  of  nitrate  of  silver, 
and  it  is  from  this  stage  that  the  plate  must  be  kept  in 
the  dark  or  in  a  room  to  which  only  such  light  is  admitted 
as  is  unable  to  act  upon  it.  The  nitrate  of  silver  is 
soluble  in  water  and  so  also  is  the  iodide  that  was  added 
to  the  collodion,  but  as  soon  as  these  two  substances 
come  together  in  a  liquid,  iodide  of  silver  is  foimed  and 
deposited  as  a  fine  yellow  powder,  because  it  is  not 
soluble  in  water.  It  will  be  seen  that  as  the  iodide  added 
to  the  collodion  mixed  thoroughly  with  it,  it  must 
thoroughly  permeate  the  film  on  the  plate,  and  as  the 
silver  nitrate  solution  soaks  into  the  film  the  iodide  of 
silver  is  produced  throughout  its  substance  the  silver 
compound  is  in  the  film  not  on  it.  After  three  or  four 
minutes  the  plate  is  withdrawn  from  the  nitrate  of  silver 
solution  (called  the  “  silver  bath  "),  allowed  to  drain  a  little, 
and  at  once  put  into  its  carrier  and  exposed  in  the 

camera.  . 

It  will  be  observed  that  the  plate  is  wet  with  the 

solution  of  nitrate  of  silver  when  it  is  being  exposed, 
and  indeed  the  plate  is  wet  all  the  time  until  it  is  ready 
for  the  final  operations  of  drying  and  varnishing.  Hence 
this  is  often  called  the  “wet  plate  process."  It  is  essential 
for  the  proper  action  of  light  that  the  plate  have  nitrate 
of  silver  upon  it,  and  if  it  were  allowed  to  dry  this  salt 
would  crystallise,  and  the  crystals  being  deposited  here 
and  there  irregularly  would  cause  an  uneven  action. 
The  wetness  is  therefore  a  matter  of  necessity  and  not  of 
convenience.  After  the  exposure  the  plate  is  taken  to 
the  dark  room,  removed  from  its  carrier,  and  a  suitable 
quantity  of  the  developer  is  poured  upon  its  surface  so 


Photography  before  Gelatine 

that  it  mixes  with  the  silver  nitrate  solution  with  which 
it  is  already  wet.  The  developer  may  be  either  pyrogallic 
acid  or  ferrous  sulphate,  but  in  either  case  its  action  would 
be  too  vigorous  alone  and  therefore  it  is  made  slower 
and  more  under  control  by  adding  a  few  drops  of  acetic 
acid  to  it.  If  the  developer  does  not  flow  evenly  on  the 
plate  a  little  alcohol  may  be  added  to  it  to  facilitate  its 
flow. 

Very  soon  after  the  developer  has  been  poured  on  to 
the  plate  the  image  begins  to  appear,  and  the  operator 
carefully  watches  the  growing  image  as  he  causes  the 
liquid  on  the  plate  to  move  a  little  to  and  fro.  When  the 
darkening  of  the  image  begins  to  lag,  the  exhausted 
solution  is  rinsed  off  the  plate,  and  a  mixture  of  some 
more  developer  with  its  acid  plus  a  little  nitrate  of  silver 
solution  is  poured  on. 

The  darkening  of  the  image  now  goes  on  again,  and 
when  the  operator  considers  that  it  is  dark  enough,  the 
plate  is  rinsed,  put  into  the  fixing  bath  to  dissolve  out 
from  the  film  all  the  silver  salt,  for  it  is  no  longer  wanted, 
washed,  dried,  and  varnished.  It  is  then  ready  to  be  used 
for  the  making  of  prints. 

The  iodide  of  silver  in  this  case  is  the  sensitive  sub¬ 
stance.  The  amount  of  light  that  gains  access  to  it  while 
the  plate  is  in  the  camera  produces  no  visible  change, 
but  it  is  certain  that  the  light  does  produce  some  change, 
because  by  the  after  treatment  those  parts  that  the  light 
has  impinged  upon  behave  in  a  different  manner  from 
those  parts  that  have  not  been  subjected  to  its  action. 
The  nature  of  the  changes  taking  place  during  develop¬ 
ment  is  not  difficult  to  understand.  If  the  developer 
proper,  the  pyrogallic  acid  or  the  ferrous  sulphate,  were 
added  to  a  solution  of  nitrate  of  silver,  metallic  silver 
would  at  once  be  separated  from  the  liquid  as  a  fine 
dark  powder,  and  in  time  this  would  settle  to  the  bottom 
of  the  vessel.  By  the  addition  of  a  drop  or  two  of 

93 


Photography  before  Gelatine 

acid  to  the  mixture,  the  separation  of  the  metal  would 
be  made  more  slow  and  it  might  be  a  minute  or  two 
before  any  change  began  to  be  visible.  This  is  the 
condition  of  the  liquid  that  is  on  the  surface  of  the 
exposed  plate  during  development.  It  is  just  ready  to 
deposit  metallic  silver,  but  it  is  held  back  by  the  acid. 
But  the  acid  holds  it  back  to  only  a  very  slight  extent, 
so  that  the  smallest  amount  of  disturbance  from  any 
outside  source  is  very  likely  to  upset  the  unstable 
equilibrium  of  the  solution  and  cause  metallic  silver  to 
separate  from  it.  We  have  seen  that  light  is  a  force 
and  often  causes  changes,  and  it  has  in  this  case  disturbed 
the  silver  iodide  in  the  collodion  film  on  the  plate  where 
it  has  fallen  upon  it,  and  this  disturbed  condition  of 
the  silver  compound  is  just  able  to  cause  the  deposition 
of  silver  from  the  solution.  So  silver  is  deposited  where 
the  light  has  acted,  but  not  in  the  other  parts  of  the  film. 
After  a  little  time  the  metal  begins  to  be  deposited 
from  the  developer  without  the  action  of  any  outside 
disturbing  force,  and  the  separating  metal  imparts  a 
brownish  tinge  to  the  solution.  If  this  were  allowed 
to  continue  silver  would  be  precipitated  all  over  the  plate 
and  the  negative  would  be  “fogged.”  To  avoid  this 
catastrophe,  as  soon  as  the  developing  liquid  is  seen 
to  be  beginning  to  be  coloured  it  is  thrown  off  and  a 
new  solution  is  applied. 

What  is  the  nature  of  the  disturbance  that  the  light 
has  effected  in  the  iodide  of  silver  ?  This  is  not  known. 
Various  theories  have  been  suggested  and  the  greater 
number  or  all  of  them  have  in  turn  been  cast  aside. 
We  shall  say  something  on  this  subject  when  we  discuss 
exposure  and  development  more  in  detail. 

We  have  just  described  the  production  of  a  negative 
wet  collodion  plate,  and  seen  that  in  the  end  there 

94 


on  a 


Photography  before  Gelatine 

is  a  dark  deposit  wherever  the  light  has  acted  on  the 
plate.  If  this  deposit  can  be  got  white  instead  of  grey 
or  black,  we  shall  get  at  once  on  the  plate  what  may  be 
imagined  to  be  a  white  pigment  corresponding  in  place 
and  in  quantity  to  the  whiteness  or  brightness  of  the 
object  photographed,  and  if  a  black  material  is  put  all 
over  behind  the  plate,  we  have  much  the  same  result  as 
would  be  obtained  by  an  artist  who  started  with  a  black 
surface  and  painted  on  it  in  white  pigment  a  picture  of 
the  object  before  him.  Only,  of  course,  our  photograph 
would  be  far  more  perfect  than  the  artist's  picture  and  it 
would  lack  the  personality  of  the  latter  which  is  often  so 
welcome  and  so  valuable.  This  is  a  collodion  positive. 
To  get  the  deposit  white,  some  ferrous  nitrate  and  nitric 
acid  are  used  in  the  developer  instead  of  only  ferrous 
sulphate  and  acetic  acid,  and  it  is  desirable  to  give  a 
shorter  exposure,  so  that  the  deposit  may  not  be  so  dense 
as  to  give  a  flat  monotonous  appearance  even  in  the 
highest  lights.  The  black  backing  may  be  of  black  velvet 
or  a  black  varnish  painted  on.  If  the  backing  is  put  on 
the  film  side  of  the  plate  so  that  the  photograph  is  looked 
at  through  the  glass,  the  picture  is  seen  correctly ;  but  if 
the  backing  is  put  on  the  glass  side  so  that  the  film  is 
towards  the  spectator,  the  image  is  laterally  reversed,  as 
a  Daguerreotype  is  when  taken  without  the  use  of  a  mirror 
or  reflecting  prism. 

A  collodion  positive  if  well  made  is  a  very  beautiful 
production,  and  some  have  said  that  it  may  surpass  the 
finest  Daguerreotype.  This  of  course  is  largely  a  matter 
of  skill  in  the  worker,  but  there  is  one  circumstance  that  is 
in  their  favour  as  compared  with  Daguerreotypes,  namely, 
that  they  have  not  that  troublesome  brightness  of  the 
polished  metal  that  renders  it  necessary  to  turn  a  Daguerreo¬ 
type  about  until  the  reflection  is  got  rid  of  before  it  can 

95 


Photography  before  Gelatine 


be  properly  seen.  The  collodion  positive  is  the  kind  of 
photograph  supplied  by  the  peripatetic  photographer  who 
gives  you  a  completed  and  framed  picture  while  you  wait. 
The  whole  process  can  be  done  in  two  or  three  minutes, 
for  the  thin  film  of  collodion  may  be  rapidly  washed  and 
if  necessary  dried  by\warming  the  plate,  without  fear  of 
damage.  A  “ferrotype”  or  “tintype”  is  a  collodion 
positive  made  on  a  piece  of  thin  enamelled  iron  which 
itself  constitutes  the  black  backing  necessary.  In  this 
case,  of  course,  the  photograph  must  be  laterally  reversed 
unless  a  mirror  is  used. 

The  necessity  for  having  the  plate  wet  and  wet  with 
a  solution  of  nitrate  of  silver,  when  working  the  collodion 
wet  plate  process,  has  been  pointed  out.  The  disadvan¬ 
tages  of  this  necessity  are  numerous  and  can  hardly  be 
appreciated  by  one  who  has  not  himself  performed  the 
necessary  operations.  Consider  a  photographer  working 
at  home  in  his  own  studio  and  about  to  take  a  portrait. 
After  arranging  the  sitter  and  getting  everything  con¬ 
nected  with  the  subject  ready,  the  photographer  must 
retire  to  the  dark  room  to  prepare  the  plate.  When  it  has 
been  in  the  nitrate  of  silver  solution  long  enough  it  has  to 
be  transferred  while  wet  and  dripping,  for  it  must  not  be 
drained  too  long,  to  the  back  portion  of  the  camera,  that 
is  made  removable  for  the  purpose  of  conveying  the  plate 
in  darkness  from  the  dark  room  to  the  camera  in  the 
studio.  It  is  hardly  possible  to  prevent  the  silver  solution 
from  getting  on  to  the  fingers  and  on  to  some  parts  of  the 
camera.  It  very  speedily  blackens  the  fingers  and  in  time 
it  rots  the  woodwork.  In  development  there  is  more 


\ 


silver  solution  liable  to  get  on  to  the  hands,  so  that  even 
the  most  careful  and  dexterous  worker  could  hardly  avoid 
justifying  the  description  of  photography  as  the  “  black 
art.”  But  if  a  photograph  had  to  be  taken  away  from 

96 


1 


! 

1 


! 

! 

1 

f 

. 


Photography  before  Gelatine 

home  there  were  other  and  greater  disadvantages.  The 
plates  had  to  be  prepared  and  developed  on  the  spot,  and 
unless  they  were  exceedingly  small  this  necessitated  the 
carrying  of  large  bulks  of  solutions  and  also  a  “tent”  to 
work  in,  a  tent  of  special  construction  to  exclude  the 
light.  The  tent  might  be  only  large  enough  to  put  the 
worker’s  head  and  hands  in,  instead  of  his  whole  body, 
but  still  it  had  to  be  of  considerable  size  to  permit  of 
the  various  manipulations  necessarily  done  in  the  dark. 
The  most  modest  outfit  for  outdoor  work  would  therefore 
generally  be  an  arrangement  on  wheels  and  require  the  aid 
of  at  least  one  assistant. 

A  “dry  plate,”  that  is  a  plate  that  could  be  prepared 
at  home,  and  that  would  keep  in  good  condition  long 
enough  for  it  to  be  taken  where  desired  for  exposure  and 
then  brought  home  for  development,  was  clearly  a  great 
desideratum.  The  wet  collodion  plate  could  not  be  dried, 
for  then  the  nitrate  of  silver  would  crystallise  on  its 
surface  and  render  it  useless,  and  the  silver  salt  could 
not  be  washed  away  to  permit  of  its  drying,  for  that  would 
render  the  plate  very  much  less  sensitive.  The  search 
therefore  was  for  some  substance  that  would  effectively 
replace  the  nitrate  of  silver  on  the  plate,  not  crystallise 
when  it  was  dried,  and  at  the  same  time  form  a  protective 
film.  Many  substances  were  found  to  serve  this  purpose 
to  a  greater  or  less  extent,  and  the  want  of  a  discriminating 
knowledge  as  to  their  constituents  led  various  persons 
from  time  to  time  to  succeed  with  and  to  recommend  such 
things  as  tea,  coffee,  wine,  stout,  porter,  gin  and  water, 
ale,  jelly,  raspberry  syrup,  raisins,  eggs,  rice,  tapioca, 
honey,  sugar  candy,  gallic  acid,  tannin,  gelatine,  &c. 

These  collodion  dry  plates  were  prepared  as  described 
for  a  wet  plate,  but  allowed  to  remain  rather  longer  in  the 
silver  bath  to  ensure  a  complete  action,  then  instead  of  at 

97  G 


Photography  before  Gelatine 

once  exposing  them,  the  plates  were  washed,  coated  with 
the  preservative  and  dried.  The  photographer  would  as 
a  rule,  prepare  as  many  as  he  wanted  the  day  before  they 
were  to  be  used.  Such  plates  were  generally  less  sensitive 
than  a  wet  plate  and  might  require  as  much  as  thirty  times 

as  long  an  exposure.  .  ,,  , 

It  will  be  observed  that  in  the  earliest  silver  methods, 

Wedgwood’s  and  Fox  Talbot's,  the  paper  that  was  to  carry 
the  silver  compound  was  impregnated  alternately  with 
the  two  substances  that  would  produce  the  desired  silver 
salt  When  albumen  or  collodion  was  used  the  pho  o- 
grapher  himself  prepared  the  film  at  the  time  required, 
and  he  added  one  of  the  two  necessary  compounds,  e 
iodide  of  potassium,  for  example,  to  the  bulk  of  the 
material  of  which  he  prepared  the  film.  After  the  glass 
was  coated  and  the  film  produced,  it  only  remained  to 
immerse  the  film  in  the  other  solution,  that  is,  the  nitrate 
of  silver,  to  produce  the  required  silver  compound.  It 
seems  a  most  obvious  step  further,  to  add  the  nitrate  o 
silver  solution  to  the  film-producing  substance  while  it 
is  still  in  bulk,  so  that  the  sensitive  silver  salt  may  be 
produced  in  the  bulk  of  the  material  instead  of  in  each 
plate  separately.  This  clearly  means  a  saving  of  labour, 
and  a  simplification  of  work  at  the  time  when  the  photo¬ 
graph  has  to  be  taken,  for  as  soon  as  the  material  was 
poured  on  to  the  glass  and  the  film  produced,  the  sensitive 
compound  would  be  there  without  further  manipulation, 
and  the  need  for  the  silver  bath  with  all  its  attendant 
difficulties  and  unpleasantnesses  would  be  done  away  with. 
There  were  several  attempts  to  bring  about  this  simpli¬ 
fication,  but  it  seems  that  the  first  practically  successful 
formula  was  published  by  B.  J.  Sayce  and  W.  B.  Bolton 
in  1864,  and  shortly  after  improved  and  modified  in  many 

ways  by  these  and  other  workers. 

98 


Photography  before  Gelatine 

A  liquid  prepared  as  stated,  which  contains  in  it  the 
sensitive  compound  and  also  the  material  that  is  to  form 
the  film,  is  called  an  “emulsion,"  in  such  a  case  a  “photo¬ 
graphic  emulsion,"  because  the  word  emulsion  is  of  general 
applicability  and  merely  indicates  a  milky  liquid.  In  milk 
it  is  the  little  globules  of  butter-fat  that  render  the  liquid 
non-transparent  or  “  milky,"  while  in  a  photographic 
emulsion  it  is  the  little  particles  of  silver  salt  that  cause 
a  similar  appearance. 

Messrs.  Sayce  and  Bolton  found  that  bromide  of  silver 
was  better  than  the  iodide,  and  this  was  one  of  the  chief 
elements  in  their  success.  The  coated  plates  were  washed 
and  the  preservative  applied  as  if  they  had  been  prepared 
by  the  use  of  a  silver  bath.  Some  ten  years  afterwTards 
the  washing  was  done  by  Mr.  Bolton  on  the  bulk  of 
emulsion  instead  of  each  single  plate  being  separately 
washed.  This  was  another  considerable  advance,  for  every 
operation  that  can  be  carried  out  on  the  bulk  of  material 
instead  of  upon  the  individual  plate,  means  not  only  a 
saving  of  time  and  an  economy  of  material,  but  a  greater 
uniformity  in  the  properties  of  the  several  plates  prepared 
from  the  batch  of  emulsion.  The  washing  is  effected  by 
adding  water  a  little  at  a  time  to  the  emulsion,  which  it 
will  be  remembered  is  a  solution  of  pyroxyline  (gun-cotton) 
in  a  mixture  of  alcohol  and  ether  containing  also  the  sensi¬ 
tive  silver  salt,  and  as  the  pyroxyline  is  not  soluble  in 
water  it  is  gradually  thrown  out  of  solution  in  a  curdy 
form,  but  still  retains  the  silver  salt.  Or  the  alcohol  and 
ether  may  be  evaporated  away,  leaving  the  non-volatile 
constituents  in  the  solid  form.  By  whatever  means  the 
solvents  are  got  rid  of,  the  solid  residue  is  washed  in  water, 
pried,  and  then  redissolved  in  alcohol  and  ether. 

Collodion  processes  of  photography  are  still  employed 
o  a  considerable  extent  in  trade  works,  because  they  offer 


Photography  before  Gelatine 

many  special  advantages,  and  the  bulk  of  the  apparatus 
required  and  the  extra  manipulations  become  negligi 
in  such  establishments  when  economy  and  efficiency  are 
considered.  But  the  general  tendency  to  replace  collodion 
with  gelatine,  which  has  for  nearly  thirty  years  been  com¬ 
plete  so  far  as  amateurs  and  portrait  photographers  are 
concerned,  seems  to  be  at  work,  surely  if  slowly  wherever 
collodion  remains  in  use. 


ioo 


CHAPTER  VI 


THE  USE  OF  GELATINE 

We  have  seen  in  the  last  chapter  how  the  various  opera¬ 
tions  in  the  preparation  of  sensitive  plates  suitable  for 
getting  negatives  on,  were  gradually  transferred  from  the 
individual  plates  to  the  bulk  of  material  with  which  the 
glass  was  to  be  coated.  Each  step  in  this  direction  gets 
nearer  to  the  possibility  of  making  plates  on  the  large 
scale  commercially  and  so  saving  the  photographer  him- 
self,  not  only  from  a  great  deal  of  trouble,  but  also 
from  much  anxious  thought  at  the  very  time  when  his 
attention  is  needed  for  other  matters.  Another  advan¬ 
tage  of  the  production  on  a  large  scale  is  uniformity 
and  therefore  reliability,  for  each  individual  plate  is  no 
longer  a  separately  made  article  with  all  possibilities  of 
failure  on  its  own  account,  but  it  is  one  of  many  all  as 
alike  as  they  can  be  made,  and  if  a  few  of  a  large  batch 
prove  satisfactory,  the  probability  is  that  the  remainder 
are  equally  good. 

About  the  time  that  we  are  now  considering,  a  few 
years  before  and  after  1870,  the  dried  collodion  emulsion 
was  an  article  of  commerce  and  dry  collodion  emulsion 
plates  were  also  prepared  commercially,  though  on  what 
would  now  be  considered  as  an  exceedingly  small  scale, 
i  Dry  collodion  plates  can  still  be  purchased,  or  at  least 
they  were  on  the  market  a  very  short  time  ago,  but 
their  use  is  restricted  almost  entirely  to  the  preparation 
of  lantern  slides. 

On  the  part  of  photographers  as  a  body,  there  was 

101 


The  Use  of  Gelatine 

for  long  after  this  time  a  distinctly  adverse  disposition 
with  regard  to  commercially  made  plates  and  other  sensi¬ 
tive  material.  There  was  a  kind  of  feeling  that  a  Phot°" 
grapher  ought  to  prepare  his  own  plates  and  that  he 
was  hardly  fit  to  associate  as  a  photographer  with  other 
photographers  unless  he  did  so.  It  took  quite  a  long  ime 
for  the  very  real  advantage  in  the  use  of  a  commercially 
made  article  to  be  appreciated.  It  is  difficult  to  under¬ 
stand  this  conservative  attitude  at  the  present  time,  when 
besides  the  advantages  already  mentioned,  the  gain  in  the 
quality  of  the  materials  is  so  obvious  when  a  staff  ot 
workers  can  devote  their  whole  attention  to  the  making 
of  them,  and  the  large  output  renders  it  profitable  to  adopt 
elaborate  precautions  that  would  have  been  quite  impos¬ 
sible  with  the  man  who  made  only  the  few  plates  that 
he  required  for  his  own  use.  But  it  must  be  remembered 
that  the  perfection  of  the  processes  for  the  commercial 
preparation  of  sensitive  material  was  very  gradual  and 
the  methods  at  first  employed  did  not  differ  from  those 
adopted  by  the  individual  photographer,  and  it  is  possible 
that  the  man  preparing  plates  for  his  own  use  tended  to 
be  a  little  more  careful  than  he  who  prepared  them  for 
sale.  This  possibility  is  not  a  matter  of  photography  but 
of  human  nature,  for  however  honest  and  generous  one 
may  be,  most  of  us  are  willing  to  take  a  little  more  troub  e 
for  ourselves  than  for  other  people.  But  the  advantages 
of  manufacture  on  the  large  scale  gradually  became  so 
obvious  that  since  some  twenty  years  or  so  ago  a  photo¬ 
grapher  would  no  more  think  of  making  his  own  plates 
than  of  making  his  own  cameras  and  lenses. 

Except  for  a  few  special  purposes  generally  in  con¬ 
nection  with  trade  work,  collodion  is  now  altogether 
supplanted  by  gelatine  as  the  medium  in  which  the  sensi¬ 
tive  salt  is  held.  It  was  collodion  that  popularised  photo¬ 
graphy,  so  that  every  one  considered  it  his  duty  to  have 
his  portrait  taken  from  time  to  time  and  give  copies  to 
^  102 


The  Use  of  Gelatine 

his  friends,  and  tourists  were  not  satisfied  unless  they 
brought  home  photographs  of  the  objects  of  interest 
associated  with  the  places  that  they  visited.  But  it  was 
gelatine  that  carried  the  development  a  stage  further,  and 
convinced  millions  of  people  how  easy  it  would  be  to 
make  their  own  photographs.  This  led  to  the  production 
of  a  type  of  amateur  photographer  quite  different  from 
the  original,  who  thought  no  trouble  too  great  and  no 
process  too  messy,  so  long  as  he  succeeded.  The  new 
type  knew  nothing  whatever  about  photography  and  cared 
nothing  about  it,  he  merely,  as  the  Kodak  Company  ex¬ 
pressed  it,  “pressed  the  button,"  and  all  the  photographic 
work  was  carried  out  for  him  by  a  trade  firm.  Between 
these  two  extremes  there  is  now  represented  every  degree 
of  interest  and  enthusiasm. 

But  the  introduction  of  gelatine  was  like  almost  all 
other  improvements  in  photographic  work,  very  gradual. 
It  was  not  so  much  that  gelatine  as  a  medium  was  a 
novelty,  but  rather  what  could  be  done  with  it.  As 
collodion  took  years  to  supplant  the  Daguerreotype  pro¬ 
cess,  so  gelatine  took  many  years  to  oust  collodion. 
Even  when  the  superiority  of  a  new  method  is  demon¬ 
strated,  the  actual  workers  who  have  become  skilled  in 
the  older  method,  are  not  very  ready  to  change  their 
habits  and  learn  a  new  process. 

It  was  in  1871  that  Dr.  R.  L.  Maddox  published  a 
formula  for  preparing  a  gelatine  emulsion  of  silver  bromide. 
It  was  a  very  imperfect  process  and  hardly  comparable 
with  the  gelatino-bromide  process  of  to-day.  Others  were 
at  work  with  gelatine  and  sought  to  make  the  use  of  it 
a  commercial  success,  but  they  treated  it,  as  was  natural, 
after  the  manner  of  collodion,  and  sought  to  sell  the 
emulsion  or  the  dried  emulsion — the  pellicle  ;  it  was  not 
yet  that  dry  gelatine  plates  ready  for  use  were  offered 
for  sale.  The  greater  sensitiveness  of  some  of  these 
newer  preparations,  when  compared  with  collodion  plates, 

103 


The  Use  of  Gelatine 

proved  a  disadvantage  rather  than  otherwise,  as  it  caused 
many  who  tried  them  to  fail  on  account  of  the  disas¬ 
trously  long  exposures  that  they  gave. 

Various  improvements  were  effected  in  the  use  of 
gelatine,  but  the  especial  feature  of  the  modern  dry  plate, 
that  is  its  very  great  sensitiveness  as  compared  with  the 
plates  prepared  by  any  earlier  process,  dates  from  1878. 

It  was  in  this  year  that  Mr.  Charles  Bennett,  a  member 
of  the  well-known  firm  of  hatters  and  an  amateur  photo¬ 
grapher  of  the  early  and  enthusiastic  type,  exhibited  some 
photographs  taken  with  extraordinarily  short  exposures. 
His  method  was  to  keep  the  emulsion  warm,  or  rather 
to  prevent  it  getting  quite  cold  (about  qo°  F.),  for  a  few 
days,  at  longest  about  a  week.  Dry  plates  had  been 
made  and  sold  before  this,  for  towards  the  end  of  1877 
they  were  advertised  for  sale  by  Mr.  Kennett  and  also 
by  Wratten  and  Wainwright,  who  supplied  both  collodio- 
bromide  and  gelatino-bromide  dry  plates.  But  at  this 
time  a  wel-lknown  authority  writes  :  “  Probably  the  final 
test  of  the  excellence  of  a  dry  plate  process  will  consist 
in  its  fitness  for  commercial  sale,  ready  prepared  for 
use,  with  some  certainty  on  the  part  of  purchasers  that 
reasonable  excellence  may  be  secured  in  the  use  of  such 
plates.  This  is  a  test,  which,  judging  from  the  reports' 
of  the  users  of  commercial  dry  plates,  either  no  process, 
or  no  manipulation  of  any  process,  has  yet  stood  satis¬ 
factorily."  A  year  later,  that  is  after  Bennett's  method 
of  getting  increased  sensitiveness  had  been  published,  we 
find  an  experienced  photographer  who  used  both  collo¬ 
dion  and  gelatine  emulsions,  always  preferring  collodion 
as  being  more  easy  and  certain  to  work ;  which  of  course 
was  natural,  as  the  experience  with  collodion  was  con¬ 
siderable  and  gelatine  was  comparatively  new.  A  noted 
photographer  found  Swan’s  gelatine  dry  plates  to  require 
from  a  half  to  a  sixth  of  the  exposure  of  a  wet  collodion 
plate,  while  Wratten  and  Wainwright  said  that  their 

104 


E.  Scamell 

“  Paper ! ” 

One  of  a  series  of  records  of  “  London  Street  Cries,”  some  of  which,  that  were 
common  years  ago,  have  now  passed  away. 


The  Use  of  Gelatine 

gelatine  dry  plates  needed  only  one-fifteenth  of  the  ex¬ 
posure  of  a  collodion  plate,  and  in  a  poor  light  only  one- 
fortieth. 

Thus  dry  plates  were  gradually  being  improved,  and  in 
1879  Mr.  Joseph  Paget  offered  a  prize  of  .£50  for  the  best 
dry  plate  process  ;  the  Paget  Prize  Plate  Company  was  the 
result  of  this  competition.  Before  the  close  of  1880  there 
were  many  makers  of  gelatino-bromide  dry  plates,  the  new 
industry  had  established  itself,  and  it  only  remained  to 
perfect  it  in  details.  It  was  soon  found  that  a  few  minutes 
at  the  temperature  of  boiling  water  was  as  effective  in 
getting  greatly  increased  sensitiveness  as  a  week  at  a 
temperature  equal  to  that  of  a  very  hot  summer's  day  in 
England,  and  Dr.  Monckhoven  found  that  by  adding 
ammonia  to  the  emulsion  the  heating  might  be  dispensed 
with  altogether. 

Although  by  this  time,  1880,  the  modern  gelatino- 
bromide  dry  plate  was  being  prepared  commercially  by 
many  firms,  they  were  prepared  in  much  the  same  way 
that  an  individual  experimenter  would  make  a  dozen  or 
two  for  his  own  use.  But  the  possibility  of  buying  plates 
which  needed  absolutely  no  preparation  to  fit  them  for 
use,  plates  that  could  be  taken  from  their  package  and  put 
straight  into  the  camera,  and  that  would  keep  in  good 
condition  for  weeks  if  not  months  either  before  exposure 
or  between  exposure  and  development,  soon  began  to 
cause  a  great  addition  to  the  number  of  those  who  prac¬ 
tised  photography.  The  greatly  increased  sensitiveness 
of  them  as  compared  with  collodion  soon  made  them  a 
necessity  for  the  professional  portrait  photographer,  for 
the  possibility  of  getting  a  sufficient  exposure  in  four 
seconds  instead  of  forty  or  sixty,  was  too  great  an  ad¬ 
vantage  both  to  the  photographer  and  to  his  sitters  to 
be  neglected. 

The  demand  for  dry  plates  therefore  gradually  in¬ 
creased,  and  the  need  for  machinery  to  take  the  place  of 

!°5 


The  Use  of  Gelatine 

hand  work  soon  began  to  be  felt.  But  there  was  no 
machinery  available  for  cleaning  glass  plates  and  coat¬ 
ing  them  with  a  warm  gelatinous  mixture.  Of  course 
inventors  were  soon  at  work,  but  only  experience  could 
decide  as  to  the  efficiency  of  the  apparatus  devised.  In 
October,  1888,  the  manager  of  one  of  the  largest  plate- 
making  companies  wrote  to  the  author :  “  Up  to  t  le 
present  all  (our  plates)  have  been  hand-coated  for  the 
simple  reason  that  we  have  not  seen  any  machine  which 
worked  to  our  entire  satisfaction  until  quite  lately.  Sue 
a  machine  is  now  completed  for  us  and  we  hope  to  have 
it  in  work  in  about  a  month.  It  coats  very  perfect  y, 
and  has  at  least  two  very  desirable  qualities,  namely, 
it  puts  a  measured  quantity  of  emulsion  upon  each  plate 
and  none  whatever  on  the  back."  Coating  by  hand 
means  that  the  warm  emulsion  was  poured  from  a  tea¬ 
pot  on  to  the  plate  held  in  the  hand,  caused  to  flow  all 
over  the  plate  by  tipping  it  as  required  or  by  the  use  of 
a  class  rod,  that  the  plate  was  then  put  on  a  level  stand 
until  the  emulsion  had  cooled  and  “set,"  and  then  dried 
A  coating  machine  would  coat  more  than  a  thousand 

large  plates  in  an  hour. 

Gelatine  plates  at  the  present  time  are  all  made  in 
factories  built  for  the  purpose.  Every  precaution  is 
taken  to  avoid  dust,  for  dust  on  the  plates  means  specks 
in  the  negative  and  this  means  spots  on  the  print.  For 
this  reason  large  towns  are  avoided,  and  all  the  air  that 
enters  those  parts  of  the  factory  concerned  is  filtered 
Some  idea  of  the  effect  of  the  filtration  may  be  obtained 
from  the  fact  that  at  a  factory  away  in  the  country,  it 
was  observed  that  some  of  the  men  engaged  in  changing 
the  filtering  cloths  were  always  ill  after  the  operation, 
which  was  done  once  in  three  months.  As  soon  as  the 
connection  between  the  filter  cloth  changing  and  the 
illness  was  discovered,  the  men  were  provided  wi 
suitable  respirators  and  then  their  health  did  not  suffer. 


The  Use  of  Gelatine 

This  indicates  a  very  considerable  accumulation  of  patho¬ 
genic  germs  on  the  filters  as  well  as  all  the  dust  and  soot  and 
obvious  “dirt.”  But  even  this  is  not  sufficient  precaution, 
and  it  is  desirable  to  arrange  so  that  there  shall  be  no 
crevices  or  spaces  that  cannot  be  easily  cleaned  and  that 
might  therefore  allow  an  accumulation  of  dust,  and  to 
wash  the  floor  every  morning.  It  will  be  understood  that 
this  indicates  the  general  character  of  the  precautions 
necessary,  and  that  these  will  vary  somewhat  according 
to  needs  of  different  localities  and  the  ideas  of  different 
managers. 

Considerable  difficulty  was  experienced  at  first  in 
getting  suitable  glass.  Plate  glass  would  be  much  too 
costly,  and  sheet  glass  with  a  very  uneven  surface  or  with 
specks  or  other  irregularities  in  it  is  obviously  unsuitable. 
Good  sheet  glass  being  obtained,  it  is  cut  into  pieces 
of  the  required  size  and  any  pieces  that  show  flaws  are 
rejected.  The  pieces  of  glass  are  then  fed  into  a  machine 
which  carries  them  between  moving  brushes,  over  which 
a  suitable  cleaning  liquid,  such  as  a  solution  of  carbonate 
of  soda,  is  constantly  flowing.  They  are  carried  through 
plain  water  to  complete  the  washing,  and  then  generally 
receive  a  coating  of  a  dilute  solution  of  gelatine  containing 
chrome  alum,  so  that  when  it  dries  the  plate  is  covered 
with  a  very  thin  film  of  insoluble  gelatine.  This  sub¬ 
stance  enables  the  emulsion  film  to  adhere  more  securely 
to  the  glass.  Without  it  the  film  might  swell  up  away 
from  the  glass  in  blisters,  or  t( frill"  at  the  edges,  or  when 
dry  it  might  come  away  from  the  glass  or  be  peelable  from 
it  so  readily  as  to  be  unsafe. 

The  prepared  glasses  are  now  fed  into  the  coating 
machine,  and  are  carried  along  on  a  continuous  band 
under  the  trough  that  deposits  upon  them  a  liquid  sheet 
of  the  warm  gelatinous  emulsion,  and  they  are  carried 
along  continuously  through  a  tunnel  or  the  equivalent 
which  is  cooled  by  means  of  ice.  The  liquid  emulsion 

107 


The  Use  of  Gelatine 

here  sets  to  a  jelly,  and  it  is  desirable  that  it  shall  set  as 
quickly  as  possible  to  prevent  the  solid  particles  of  silver 
salt  from  settling  down  to  the  lower  surface  of  the  him. 
The  plates  are  then  stacked  in  racks  and  taken  to  t  e 
drying  chambers  where  they  are  maintained  at  a  warm 
temperature,  which  however  is  not  warm  enough  to  run 
any  risk  of  melting  the  jelly.  A  current  of  filtered  air 
passes  between  the  plates  and  through  the  chambers  until 
the  coating  is  dry.  The  plates  are  then  removed  in  their 
racks  to  be  cut  up,  if  the  smaller  sizes  such  as  half-plate 
and  under  are  required.  Mechanical  arrangements  are 
employed  as  far  as  possible  for  this  cutting,  as  it  is  done  in 
the  minimum  of  dark  red  light.  The  plates  are  then 
examined,  and  those  that  show  defects  are  thrown  out  and 
the  remainder  are  packed  for  sale.  All  the  operations 
from  the  coating  with  emulsion  onwards  are  done  in  rooms 
lit  with  only  a  red  light  that  is  so  feeble  that  any  one 
a0ing  in  from  outside  would  probably  be  ten  minutes  or 
more  before  their  eyes  became  sufficiently  accustomed 
to  the  semi-darkness  to  enable  them  to  see  their  way 

about.  , 

The  emulsion  is  prepared  in  large  quantities  and  stored 

in  jars.  One  of  the  methods  of  securing  uniformity,  is  to 
make  several  batches  as  nearly  as  possible  alike  and  to 
mix  them,  so  that  the  general  average  may  vary  but 
little.  Every  ingredient  has  to  be  suitably  pure,  and  as 
an  amount  of  impurity  that  would  be  far  too  small  for 
detection  by  any  method  of  chemical  analysis  might 
interfere  vitally  with  the  quality  of  the  emulsion,  the 
suitability  of  each  constituent  is  tested  by  making  a 
small  quantity  of  emulsion  with  it,  using  other  ingredients 
of  known  quality,  and  testing  a  plate  coated  with  the 
resulting  preparation.  This  applies  not  only  to  e 
gelatine,  which  is  liable  to  vary  very  much  according 
to  the  method  of  its  preparation  and  purification,  bu 
to  the  nitrate  of  silver,  which  is  a  definite  substance 

108 


even 


The  Use  of  Gelatine 

and  being  easily  crystallisable  is  supposed  to  be  easily 
purified. 

The  principles  involved  in  the  making  of  a  gelatine 
emulsion  are  exceedingly  simple,  and  the  practice  is 
almost  as  simple  when  the  most  suitable  procedure  has 
been  established.  A  portion  of  the  gelatine  is  dissolved 
in  water  with  the  requisite  bromide  of  potassium  and 
a  little  iodide  of  potassium,  for  this  latter  is  found  to 
give  certain  desirable  qualities  that  the  bromide  alone 
is  deficient  in.  The  nitrate  of  silver  is  dissolved  in  water 
and  added  to  it.  The  insoluble  bromide  and  iodide  of 
silver  are  produced  as  a  fine  powder  which  permeates 
the  whole  solution,  and  nitrate  of  potassium  is  produced 
from  the  other  constituents  of  the  compounds.  It  is 
desirable,  or  rather  it  is  necessary,  to  get  rid  of  the 
nitrate  of  potassium  together  with  the  excess  of  the 
bromide  of  potassium  present,  and  this  is  accomplished 
by  cooling  the  emulsion  until  it  becomes  a  jelly  and 
washing  the  jelly  in  repeated  changes  of  water.  The 
rest  of  the  gelatine  is  then  added  and  the  whole  melted 
up  together.  It  is  well  at  some  stage  of  the  operation 
to  filter  the  emulsion,  as  some  of  the  silver  salt  is  likely 
to  clot  together  and  form  large  particles  which  would 
give  an  irregular  granularity  to  the  negative  made  on 
the  plate.  For  getting  the  enhanced  sensitiveness  the 
emulsion  is  heated  as  necessary  before  the  main  quantity 
of  gelatine  is  added  to  it,  or  ammonia  is  used  instead 
of  the  heating,  or  a  combination  of  the  two  methods 
may  be  employed.  No  maker  publishes  exactly  the 
method  that  he  adopts,  and  probably  there  are  little 
differences  in  the  various  factories,  but  so  far  as  the 
user  of  the  plates  is  concerned  these  differences  are  not 
|  very  great.  Some  makers  put  more  silver  salt  into  their 
emulsions  than  others,  some  put  thicker  films  upon  their 
plates,  some  use  harder  gelatines,  and  there  are  other 
differences  referring  to  the  gradation  that  they  yield  that 

109 


The  Use  of  Gelatine 

will  be  subsequently  considered.  But  the  main  difference 
to  the  ordinary  user  is  in  the  sensitiveness,  and  this  is 
not  a  difference  as  between  one  maker  and  another  so 
much  as  between  the  various  kinds  of  plate  that  each 
maker  produces. 

It  might  perhaps  be  thought  that  the  greater  the  quan¬ 
tity  of  silver  salt  and  the  thicker  the  film  the  better  the 
plate  if  the  other  qualities  remain  the  same.  But  no 
such  broad  generalisation  is  justifiable.  It  is  not  un¬ 
usual  for  an  operator  to  be  exposing  plates  all  clay,  or 
as  long  as  the  daylight  lasts,  and  then  to  develop  his 
plates  before  going  home.  Plates  with  thick  films  take 
longer  to  wash  than  those  with  thin  films,  and  if  therefore 
he  gives  the  same  time  to  both  sorts,  the  thinner  films 
will  perhaps  be  well  washed  while  the  thicker  films  will 
be  imperfectly  washed  and  suffer  afterwards  on  that 
account.  But  will  not  the  image  on  the  negative  be 
better  for  a  liberal  supply  of  silver  salt,  it  may  be  asked. 
Even  this  does  not  follow.  Many  portraits  are  taken, 
especially  of  ladies  and  children,  in  which  the  whole 
subject  is  very  light  and  shows  very  little  contrast.  1 
there  is  only  a  small  difference  between  the  darkest  and 
the  brightest  parts  of  the  subject,  whatever  it  is,  then 
a  similarly  small  difference  is  all  that  is  needed  in  the 
negative  and  more  than  sufficient  silver  salt,  of  course 
allowing  a  good  margin,  would  only  be  in  the  way.  On 
the  other  hand,  a  brilliantly  lit  landscape  with  a  dark 
foreground,  or  an  interior  of  a  building  in  which  a  part 
is  well  lit  and  a  part  in  deep  shadow,  needs  a  richly 
coated  plate  to  do  it  justice.  Hence  it  is  desirable  to 
prepare  plates  of  different  kinds  for  different  purposes, 
as  is  customary,  and  it  is  well,  where  there  is  any  doubt, 
to  select  a  plate  with  a  generous  coating  and  make  due 
allowance  for  the  extra  time  necessary  for  its  successful 

treatment.  , 

Thus  we  have  traced  the  very  commencement  ot  tne 

no 


The  Use  of  Gelatine 

dry  plate  industry  and  to  a  certain  extent  its  development. 
As  the  practice  of  photography  has  become  increasingly 
popular  and  the  applications  of  it  have  increased,  the 
demand  for  gelatino-bromide  dry  plates  has  grown 
enormously.  The  number  and  the  size  of  the  factories 
have  increased,  and  the  author  has  been  assured  by  one  who 
is  well  qualified  to  know  that  the  number  of  plates  coated 
every  day  would  now  have  to  be  counted  in  millions. 
There  does  not  appear  to  be  any  method  of  getting  exact 
figures,  but  certainly  it  is  remarkable  that  an  industry  only 
thirty  years  old  should  have  grown  at  such  a  rate  as  it  has 
and  is  still  continuing  to  grow. 

With  the  introduction  of  systematised  labour  and 
machinery,  and  doubtless  also  the  competition  between 
makers,  the  cost  of  photographic  plates  has  gradually  been 
reduced  ;  and  if  there  is  added  to  this  the  fact  that  at  each 
change  the  intrinsic  value  of  the  plates  has  been  reduced, 
we  get  some  remarkable  figures  by  tracing  the  cost  of 
photographic  materials  during  the  last  seventy  years. 

The  price  of  plates  has  always  been  very  nearly  pro¬ 
portional  to  their  size  ;  it  is  sufficient  therefore  to  consider 
only  one  size  in  order  to  get  a  rough  idea  of  the  expenses 
attending  the  practice  of  the  art  in  its  various  stages  of 
development.  The  quarter-plate,  4^  x  3^  inches,  dates  back 
to  the  time  of  the  Daguerreotype.  One  dozen  silvered 
copper  plates  for  this  process  cost  thirty  or  thirty-two 
shillings  if  of  the  best  quality,  and  inferior  kinds  were 
supplied  down  to  as  low  a  price  as  ten  shillings.  The 
amateur  who  was  content  with  even  this  modest  size  must 
have  found  photography  a  costly  pursuit  when,  instead  of 
as  we  do  paying  a  penny  for  a  plate  all  ready  for  exposure  in 
the  camera,  he  had  to  pay  half-a-crown  for  the  plate,  which 
he  had  himself  to  prepare  by  means  of  chemicals  and  special 
apparatus.  But  if  he  were  content  with  paper  negatives 
made  by  the  calotype  or  Talbotype  process  or  its  modifi¬ 
cations,  the  cost  was  really  very  moderate.  Expensive 

hi 


The  Use  of  Gelatine 

cameras  then  were  unknown.  Many  were  r®a*1y 
more  than  a  box  with  the  lens  at  one  end  and  an  arrange 
ment  for  carrying  the  plate  at  the  other  and  the  cost  on  y 
a  few  shillings  up  to  a  pound  or  two.  The  various  adjust- 
ments  that  we  consider  indispensable  were  o  Y  g  1  ^ 

introduced,  and  as  in  the  Daguerreotype  and  wet  collodion 
processes,  only  one  plate  could  be  dealt  with  at  a  ^une 
as  it  was  prepared,  only  one  plate  earner  or  back  was 
necessary,  and  this  for  a  single  plate  The  you  h  who 
made  his  own  camera  out  of  a  cigar-box  was  really  pro¬ 
viding  himself  with  the  same  type  of  instrument  as  was 
offered  for  sale  at  the  photographic  dealers.  But  this 
economy  in  the  cost  of  the  camera  was  pretty  well  balanced 
by  the  need  for  other  apparatus,  and  the  current  expense 

was  far  greater  than  at  the  present  time. 

The  glass  plates  used  for  the  collodion  process  were 
about  three  shillings  a  dozen  quarter-plate  size  There 
were  cheaper  plates  to  be  had  and  plenty  of  them  were 
used  but  there  was  a  measure  of  risk  in  their  want  o 
■flatness,  uneven  surface,  and  other  defects  There  was, 
however,  a  compensation  for  the  high  cost  of  g  ass  p  ates 
that  if  the  negative  was  not  satisfactory  the  plate  could  be 
cleaned  and  used  again.  At  the  present  day  it  would  cost 
more  to  clean  and  recoat  spoilt  plates  than  to  use  new 

‘^Tn  1847  hyposulphite  of  soda  was  catalogued  at  five 
shillings  a  pound.  In  1853  it  was  two  shillings  the  next 
year  Jghteenpence,  and  ten  years  afterwards  fivepence. 
At  the  present  time  it  can  be  obtained  for  less  than  two¬ 
pence  a  pound,  or  perhaps  a  little  more  if  bought  in  small 

quawe  must  not  close  our  treatment  of  the  present  subject 
without  a  reference  to  the  use  of  films  instead  of  glass 
plates  as  the  support  of  the  sensitive  layer  in  the  produc¬ 
tion  of  negatives.  The  earliest  negatives  were  made  on 
and  to  a  certain  extent  in  the  substance  of  paper.  1  h 


1 1 2 


The  Use  of  Gelatine 

irregularities  of  this  material,  as  we  have  already  pointed 
out,  were  constantly  troubling  those  who  used  it,  and  the 
introduction  of  a  separate  substance  to  carry  the  sensitive 
compound  that  should  be  free  from  the  structure  of  paper 
and  under  the  photographer’s  immediate  control,  first 
albumen  and  then  collodion  and  finally  gelatine,  was  a 
very  important  improvement.  As  the  substance  of  the 
negative,  except  the  actual  image,  was  required  to  be  as 
transparent  as  possible,  it  was  natural  that  glass  should  be 
used  as  the  support  for  the  sensitive  film.  A  glass  plate 
was  not  only  the  most  transparent  support  possible,  but, 
being  rigid,  it  was  easy  to  coat,  easy  to  support  in  the 
camera,  and  every  operation  in  the  making  of  the  negative 
and  the  getting  of  prints  from  it,  was  facilitated  by  its  use. 
Being  non-absorbent  it  could  be  easily  cleaned  and  so 
freed  from  any  impurities  that  would  incur  the  risk  of 
contaminating  the  sensitive  material.  At  the  present  day 
glass  still  remains  the  best  support  so  far  as  general 
manipulations  are  concerned,  but  it  is  heavy,  bulky,  and 
brittle,  and  in  these  details  inferior  to  the  paper  used  in 
earlier  times.  The  advantages  of  paper  were  never  for¬ 
gotten. 

A  flexible  film  is  so  superior  in  portability  to  glass,  that 
as  early  as  1854,  that  is  in  the  first  days  of  collodion  and 
while  the  Daguerreotype  process  was  still  being  worked, 
a  roller  slide  was  designed  by  A.  Melhuish,  that  is  an 
arrangement  to  slide  into  the  back  of  the  camera,  and 
containing  two  rollers  between  which  the  sensitive  film 
that  is  ready  for  exposure  is  stretched.  By  having  a  long 
band  of  film  wound  on  one  roller  it  can  be  transferred 
gradually  to  the  other  as  needed  for  bringing  a  fresh 
portion  into  position  for  exposure.  Thus  a  series  of 
almost  any  number  of  exposures  might  be  made  without 
the  need  to  use  a  dark  room  for  changing  the  sensitive 
material.  This  method  of  working  was  popularised  by 
the  Eastman  Kodak  Company  in  1885.  In  their  cameras, 

113  H 


The  Use  of  Gelatine 

the  rollers  are  detachable  bodily  from  the  apparatus,  the 
film  as  wound  upon  the  roller  by  the  makers  is  slipped  into 
its  place,  and  when  it  has  been  exposed,  the  receiving 
roller  is  turned  several  times  so  that  a  length  of  black 
paper  attached  to  the  end  of  the  film  shall  be  wound  round 
it  to  protect  it  from  the  light,  and  is  then  removed  for 

development,  &c.  . 

Many  sorts  of  films  were  devised,  some  of  them  tot- 

working  on  rollers,  others  thick  enough  to  stand  alone 
like  glass,  others  were  arranged  like  drawing  blocks,  so 
that  each  as  exposed  might  be  stripped  off  leaving  the  next 
ready.  Gelatine,  collodion,  varnish,  and  paper  in  various 
forms  and  combinations  were  used  to  make  the  films  of. 
Some  of  the  films  were  held  on  temporary  supports  such 
as  paper,  from  which  they  had  to  be  removed  after  ex¬ 
posure  and  development.  Some  had  sensitive  emulsion 
on  both  sides,  so  that  any  granularity  due  to  the  material 
of  the  film  might  not  appear  in  the  finished  negative,  as 
any  unduly  thin  or  transparent  part  would  be  neutralised 
by  the  light  passing  through  it  and  giving  a  deposit,  when 

developed,  on  the  back  film. 

The  Tollable  film  that  is  at  present  so  largely  used  is 
of  celluloid,  which  can  be  obtained  free  from  structure  and 
almost  as  transparent  as  glass.  It  is  coated  on  one  side 
with  the  sensitive  emulsion,  and  to  counteract  the  tendency 
to  curl  or  roll  up  when  wet,  a  thin  layer  of  gelatine  is  put 
upon  the  back  surface.  This  kind  of  film  was  introduced 
in  1903  by  the  Kodak  Company,  and  is  in  general  use. 


CHAPTER  VII 


THE  PLATE 

The  first  question  that  arises  in  connection  with  the  plate 
is — how  sensitive  is  it  ?  The  duration  of  the  exposure 
necessary  to  produce  the  desired  effect  must  depend  upon 
this,  and  therefore  the  degree  of  sensitiveness  of  the  plate 
must  be  known  in  some  way  or  other.  Now  sensitiveness 
is  not  an  absolute  matter,  it  may  be  compared  to  a  measure 
of  length  or  capacity,  in  order  to  express  which  it  is  neces¬ 
sary  to  adopt  a  unit.  If  we  say  of  any  object  that  it  is 
three  feet  long,  we  only  mean  that  it  is  three  times  as  long 
as  a  certain  distance  which  is  called  one  foot  and  is 
adopted  as  the  unit.  A  person  who  does  not  know  the 
length  of  one  foot,  can  ascertain  it  with  a  sufficient  accu¬ 
racy  for  practical  purposes  by  buying  a  foot  rule  at  a  tool 
shop.  A  scientific  man  who  wants  to  be  more  exact  can 
gain  access  to  a  standard  measure  which  is  carefully  pre¬ 
served  for  the  sake  of  reference.  This  standard  foot  is 
a  purely  arbitrary  measure,  it  might  just  as  well  be  a  little 
longer  or  a  little  shorter,  it  would  not  matter  at  all  what 
it  was  so  long  as  it  was  definite,  and  known  to  those  who 
wished  to  use  it.  The  French  metre  is  rather  longer 
than  our  yard  and  bears  no  simple  relationship  to  it,  but 
it  is  just  as  serviceable  on  the  Continent  as  the  yard  is 
in  England,  because  every  one  concerned  knows  what 
it  is. 

It  may  seem  at  first  sight  just  as  easy  to  fix  on  and 
adopt  a  unit  or  standard  of  sensitiveness  as  a  standard  of 
length,  but  the  two  cases  are  widely  different.  First,  it 

1 15 


The  Plate 

would  be  impossible  to  preserve  a  standard  sensitive  plate 
because  of  its  liability  to  alter.  Further,  every  time  a 
comparison  was  made  with  the  standard  plate  a  pait  of 
one  would  have  to  be  used  up,  and  this  would  necessitate 
a  number  of  standards  with  their  unavoidable  variations. 
And  there  are  still  other  difficulties.  1  he  word  sensitive¬ 
ness  implies  sensitiveness  to  something.  To  light,  may 
be  answered— but  to  what  light?  is  a  most  vital  question. 

It  is  not  uncommon  to  find  at  the  present  time  a  plate 
more  sensitive  than  another  to  daylight  but  less  sensitive 
to  gas  light.  Whatever  light  is  adopted,  the  particular 
degree  of  sensitiveness  will  apply  to  that  light  only.  All 
ordinary  lights  are  complex  mixture  of  radiations  of  many 
wave  lengths,  and  the  very  essence  of  a  standard  should 
be  simplicity.  Again,  the  way  in  which  a  plate  is  treated, 
as  in  development,  may  affect  its  sensitiveness,  and  suppose 
that  a  certain  standard  treatment  were  adopted,  and  this 
has  actually  been  proposed,  and  a  plate  was  made  which 
by  some  other  treatment  was  far  more  sensitive  than  by 
the  standard  treatment,  the  standard  sensitiveness  estima¬ 
tion  would  count  for  nothing.  Who  would  care  to  know 
that  a  plate  was  no  more  sensitive  than  another  when 
tested  by  a  “  standard  "  process,  when  it  was,  say,  three 
times  as  sensitive  if  treated  by  a  method  bettei  suited 

to  it  ? 

Sensitiveness  cannot  be  definitely  estimated  unless  the 
conditions  under  which  the  plate  is  to  be  used  are  exactly 
expressed.  Although  it  is  necessary  and  possible  to  give 
a  general  idea  as  to  whether  a  plate  is  slow,  or  rapid,  or 
extra  rapid,  &c.,  it  is  not  possible  to  do  more  than  this 
unless  the  conditions  of  its  use  are  specified.  In  com¬ 
mercial  statements  as  to  sensitiveness  daylight  is  assumed 
to  be  used  unless  the  contrary  is  distinctly  stated,  and  the 
makers  of  plates  often  express  the  sensitiveness  by  figures. 
The  figures  given  by  the  same  maker  may  be  more  or  less 
comparable  among  themselves,  but  it  does  not  at  all  follow 


The  Plate 

that  the  same  figure  or  expression  given  by  different 
makers  indicates  the  same  sensitiveness. 

The  methods  of  estimating  sensitiveness  at  present 
employed  may  be  divided  into  two  classes.  The  earliest 
is  also  the  most  obvious,  and  although  much  has  been  said 
against  it,  it  must  always  remain  of  value.  It  consists  in 
arranging  a  graduated  series  of  exposures  so  that  the  least 
shall  be  small  enough  to  give  no  result  with  the  most 
sensitive  plate  to  be  tested  and  finding  out  the  very 
smallest  exposure  that  will  give  a  result  on  each  of  the 
various  plates  to  be  tested.  If  one  plate  will  give  its  first 
indication  of  a  result  with  an  exposure  that  is  one-tenth 
of  the  exposure  necessary  to  begin  to  affect  another  plate, 
it  is  obvious  that  it  is  ten  times  as  sensitive  as  the  second 
plate. 

The  light  used  for  this  purpose  must  not  be  subject 
to  change,  therefore  no  domestic  lamp,  gas,  or  electric 
light  will  serve.  It  is  most  usual  to  employ  a  definite  and 
pure  liquid  or  solid,  or  a  pure  gas  such  as  acetylene, 
and  arrange  for  its  combustion  so  that  the  flame  shall 
always  be  of  the  same  size.  Sometimes  an  opaque  screen 
with  a  hole  in  it  is  put  in  front  of  the  flame  in  order  that 
only  that  part  of  the  flame  that  is  least  liable  to  alteration 
may  be  used.  The  series  of  exposures  may  be  made  by 
putting  the  plate  far  enough  away  from  the  illuminant  and 
uncovering  it  a  part  at  a  time  for  say  one,  two,  four,  eight, 
&c.,  seconds,  or  the  period  of  the  exposure  of  the  various 
parts  may  be  the  same,  and  the  plate  may  be  brought  nearer 
to  the  illuminant  in  order  to  increase  the  exposure  effect, 
but  it  is  usual  to  save  time  and  trouble  by  putting  it 
behind  a  screen  that  graduates  the  light.  Such  a  screen 
may  be  of  the  same  size  as  the  plate  and  have  a  series  of 
squares  upon  it  of  increasing  opacity,  or  other  means  may 
be  adopted  to  secure  a  similar  effect. 

The  drawback  to  such  methods  of  estimating  sensitive¬ 
ness  is,  that  in  ordinary  photography  the  very  thinnest 

1 17 


The  Plate 

deposits  that  can  be  produced  upon  a  plate  by  the  feeblest 
light  are  rarely  of  any  use,  because  they  are  so  thin  that 
no  difference  can  be  seen  in  the  print  between  them  and 
the  parts  where  there  is  no  deposit  at  all.  They  have,  as 
is  said,  “no  printing  value."  And  it  may  happen  that 
a  plate  that  is  considerably  more  sensitive  than  another 
by  this  method  of  estimation,  will  need  even  a  longer 
exposure  than  the  apparently  less  sensitive  plate  in  order 
to  get  an  equally  good  negative  of  an  ordinary  subject. 
The  first  plate  shows  a  long  series  of  feeble  deposits  under 
the  increasing  opacities  of  the  screen,  which  cannot  be 
utilised  in  ordinary  work. 

To  avoid  this,  Messrs.  Hurter  and  Driffield  proposed 
to  neglect  altogether  these  thin  deposits  given  by  the 
feeblest  lights  and  to  consider  only  those  densities  that 
are  of  actual  use  in  ordinary  practice.  But  although  these 
thin  deposits  are  of  very  little  if  any  use  in  the  photo¬ 
graphy  of  ordinary  subjects,  they  may  in  other  cases  be 
of  the  greatest  importance,  as  for  example  in  the  detection 
of  a  very  feebly  luminous  object,  such  as  a  nebula  or  a 
star. 

An  unqualified  statement  of  sensitiveness  can  never 
do  more  than  give  a  general  and  vague  idea  of  the  char¬ 
acter  of  the  plate.  If  more  than  this  is  wanted  the 
plates  to  be  compared  must  be  tested  under  exactly  the 
same  conditions  that  will  hold  when  they  are  to  be  used, 
and  if  not  on  the  actual  subject  that  they  are  required 
for,  at  least  on  a  subject  of  an  exactly  similar  character. 
The  way  in  which  the  comparative  sensitiveness  of  plates 
may  vary  even  when  using  the  same  kinds  of  illumina¬ 
tion,  may  be  illustrated  by  experiment.  Suppose  that 
the  object  to  be  photographed  remains  without  change, 
that  the  light  is  in  no  way  altered,  and  that  the  lens 
and  the  camera  and  their  position  relative  to  the  object 
remain  exactly  the  same,  then  if  a  certain  plate,  which 
we  will  call  A,  requires  an  exposure  of  one  second  to 


Edgar  Pickard 

The  Vaulting-Horse 

The  estimated  exposure  given  in  taking  this  photograph  is  the  one  four-hundredth 
part  of  a  second.  The  movement  here  is  comparatively  simple  as  the  athletes  body 
moves  as  a  whole. 


The  Plate 

give  the  negative  required,  while  another  plate,  which  we 
will  call  B,  needs  an  exposure  of  only  the  half  of  a  second 
to  give  the  same  kind  of  negative,  then  we  should  say 
that  the  plate  B  is  twice  as  sensitive  as  the  plate  A. 
We  may  now,  without  going  outside  the  range  of  practical 
conditions,  imagine  that  we  have  another  plate,  C,  which 
is  sufficiently  exposed  under  exactly  the  same  circum¬ 
stances  in  the  one-hundredth  part  of  a  second.  This 
plate,  C,  will  be  said  to  be  one  hundred  times  as  sensi- 
sive  as  plate  A.  As  sensitiveness  is  a  purely  practical 
matter,  we  have  now  before  us  three  plates  with  sensitive¬ 
nesses,  under  the  given  conditions  that  may  be  definitely 
expressed  in  figures  as  plate  A,  i ;  plate  B,  2  ;  and  plate 
C,  100.  Suppose  that  the  object  was  illuminated  by  a 
cluster  of  one  hundred  lamps  so  arranged  that  the  light 
from  each  shone  upon  the  object  from  the  same  distance 
and  without  hindrance,  no  lamp  hidden  or  partly  hidden 
behind  another,  we  have  by  this  arrangement  an  oppor¬ 
tunity  of  definitely  controlling  the  amount  of  light  that 
illuminates  the  object,  or,  as  we  should  usually  say,  of 
controlling  the  brilliancy  of  the  light.  It  is  perfectly 
obvious  that  if  fifty  of  the  one  hundred  lamps  are  put 
out,  there  will  remain  a  half  of  the  original  light,  and  if 
ninety-nine  of  the  lamps  are  put  out  so  that  only  one 
is  left,  the  brilliancy  of  the  light  will  be  exactly  one 
hundredth  of  the  original,  supposing  as  we  do  that  all 
the  lamps  are  of  exactly  equal  illuminating  power.  In 
this  way  we  can  easily  imagine  how  we  can  definitely 
regulate  the  brilliancy  of  the  light  that  falls  upon  the 
object. 

Let  us  imagine  that  the  experiment  is  tried  of  putting 
out  half  the  lamps,  and  doubling  the  exposure  given  to 
each  of  the  plates  in  order  to  make  up  for  the  deficiency 
of  the  light,  half  the  light  for  twice  the  time,  and  we 
shall  probably  notice  very  little  difference  in  the  com¬ 
parative  sensitivenesses  of  the  three  plates.  But  if  the 

1 19 


The  Plate 

light  is  further  reduced,  if  ten  lamps  are  employed  instead 
of  the  full  one  hundred,  and  if  the  periods  of  the  ex¬ 
posures  are  multiplied  by  ten  in  order  to  compensate 
for  the  reduced  light,  then  the  three  negatives  that  result 
will  not  be  alike,  the  slower  plates  will  appear  to  be  less 
sensitive  than  they  were  at  first.  Now  extinguish  all 
but  one  lamp  so  that  the  light  is  only  one-hundredth  as 
powerful  as  at  first,  and  make  each  of  the  exposures 
one  hundred  times  as  long  to  compensate  for  the  reduc¬ 
tion  of  the  light.  In  this  case  the  negatives  on  the 
slower  plates  will  be  obviously  under-exposed,  and  the 
most  sensitive  plate  will  perhaps  show  a  little  inclination 
in  this  direction.  Under  these  conditions  the  apparent 
sensitiveness  of  plate  C  will  have  gone  down  a  little, 
perhaps  too  little  to  notice  it,  while  the  sensitiveness  of 
plates  A  and  B  may  appear  to  be  reduced  as  much  as 
to  the  half  of  what  they  were  at  first.  That  is  as  com¬ 
pared  with  the  plate  C,  instead  of  needing  fifty  and  one 
hundred  times  the  duration  of  exposure  to  give  the  same 
kind  of  negative,  they  may  need  approximately  one  hundred 
times  and  two  hundred  times  respectively. 

It  is  therefore  just  as  true  to  say  that  plate  C  is  two 
hundred  times  as  sensitive  as  plate  A,  as  to  say  that  it 
is  one  hundred  times  as  sensitive  ;  the  fact  is  that  each 
statement  is  only  a  partial  truth,  and  that  the  compara¬ 
tive  sensitivenesses  of  the  plates  as  expressed  is  true  only 
when  the  conditions  under  which  they  were  tested  are  re¬ 
produced.  In  the  experiment  that  we  have  first  described, 
all  the  lamps  gave  an  equal  amount  of  light,  but  it  is 
clear  that  the  effective  change  produced  in  the  slower 
plates  by  a  single  lamp  was  no  more  than  half  as  much 
as  the  effective  change  that  each  individual  lamp  produced 
when  they  were  all  working  together.  It  is  as  though  the 
silver  bromide  did  not  like  being  interfered  with,  and  that 
its  power  to  resist  had  to  be  broken  down  before  it  could 
be  changed  into  the  less  stable  or  developable  condition. 

120 


The  Plate 

When  this  resistance  has  to  be  overcome  by  one  lamp 
it  is  very  obvious  in  its  magnitude,  and  uses  up  in  this 
case  as  much  as  half  of  the  power  of  the  light  given  by 
the  single  lamp. 

If  now,  while  the  struggle  between  the  light  and  the 
silver  bromide  is  going  on,  the  light  is  withdrawn,  we  might 
expect  the  silver  salt  to  recover  its  original  condition  to 
a  certain  extent,  so  that  the  light  would,  on  being  allowed 
to  resume  the  struggle,  work  at  an  additional  disadvantage, 
as  it  cannot  go  on  with  its  action  exactly  where  it  left  off. 
The  plate  will  then  appear  to  be  less  sensitive  than  before, 
because  for  the  same  effect  it  will  need  a  greater  amount 
of  light  when  this  falls  upon  it  intermittently.  This 
apparent  loss  of  sensitiveness  in  the  plate  increases,  as 
might  be  expected,  as  the  light  is  made  more  feeble  and 
as  the  number  of  stoppages  in  its  action  is  increased.  In 
one  series  of  experiments  made  by  Sir  William  Abney  in 
his  investigation  of  this  matter,  he  found  that  only 
half  the  effect  was  produced  with  exactly  the  same  total 
duration  of  exposure,  when  the  exposure  was  divided 
into  twenty-eight  thousand  separate  parts  by  the  rotation 
of  a  screen  with  holes  in  it  in  front  of  the  light.  If  we 
may  apply  the  nomenclature  of  wind  to  light,  we  might 
say  that  twenty-eight  thousand  puffs  are  only  half  as 
effective  as  one  long  blow,  although  exactly  the  same 
amount  of  light  impinges  upon  the  plate  in  the  two 
cases. 

A  given  quantity  of  light  may  have  its  effect  reduced 
not  only  by  being  made  unduly  feeble,  or  by  being  much 
subdivided,  but  also  by  being  too  intense  or  concentrated. 
A  suitable  electric  spark  will  give  a  very  powerful  light  but 
of  very  short  duration,  and  therefore  highly  concentrated, 
and  Sir  William  Abney  found  the  same  amount  of  such 
light  when  made  sixty-four  times  more  intense  but  allowed 
to  act  for  only  one  sixty-fourth  of  the  time,  produced  less 
than  one-quarter  the  effect.  In  this  case  the  more 

12 1 


The  Plate 

sensitive  plates  showed  the  greater  discrepancy,  while  with 
an  unduly  feeble  light  it  is  the  slower  plates  that  are  most 
affected  by  the  exceptional  conditions  Whatever  may  be 
the  cause,  therefore,  an  extreme  vanatton  in  the  intensity 
of  the  light  in  either  direction,  or  in  its  subdivision,  causes 
a  loss  of  its  efficiency  in  its  action  on  sensitive  plates, 
although  the  total  quantity  of  light  concerned  remai 
exactly  the  same,  and  this  loss  is  different  with  differen 

^Change  of  temperature  also  affects  the  sensitiveness  ; 
of  plates,  warming  increasing  it  and  cooling  decreasing  1. 
The  loss  of  sensitiveness  produced  by  extreme  cold,  as  by 
the  evaporation  of  liquid  air,  is  not  so  great  as  one  might 
expect  from  the  changes  that  result  from  an  alteration  of 
temperature  within  the  small  range  of  forty  or  fifty  degrees 
centigrade  that  we  are  subjected  to  during  the  changing 
seasons.  But  this  subject  needs  further  investigation. 

Thus  we  see  that  almost  every  variation  of  circum¬ 
stance  affects  sensitiveness  of  plates  and  different  plates  o 
different  extents.  Sensitiveness  is  not  an  inherent  quality 
of  the  plate,  and  it  cannot  be  expressed  by  a  definite  gure 
any  more  than  the  strength  of  a  man  can  be  so  expressed. 
We  call  one  man  strong  and  another  weak,  and  we  now 
what  is  meant.  So  we  may  say  that  one  plate  is  rapi 
and  another  slow.  We  may  say  in  general  terms  that  one 
man  is  able  to  lift  twice  as  heavy  a  weight  as  another  man. 
So  we  may  say  in  an  equally  vague  way  that  one  plate  . 
is  twice  as  sensitive  as  another.  But  it  would  be  absurd 
to  attempt  to  find  the  exact  weight  to  a  pound  that  two 
men  could  each  lift  in  order  to  express  their  relative 
powers  in  exact  figures.  If  such  an  experiment  were  made 
it  would  only  express  their  relative  strengths  under  t 
particular  conditions,  and  a  very  different  result  might  be 
obtained  under  other  conditions.  And  the  relative  sensi¬ 
tiveness  of  plates  is  just  as  indefinite. 

The  importance  of  the  exact  estimation  of  sensitiveness 

122 


The  Plate 

is  often  much  overrated  so  far  as  ordinary  work  is  con¬ 
cerned.  It  is  doubtful  whether  any  photographer  em¬ 
ploying  plates  for'  general  purposes  could  detect  a  gain 
of  25  per  cent.,  nnd  indeed  a  difference  of  50  per 
cent,  would  often  pass  unrecognised.  It  may  be  taken 
as  a  general  rule  that  if  a  photograph  shows  signs  of  too 
short  an  exposure,  it  is  not  worth  while  to  increase  the 
exposure  next  time  to  any  less  proportion  than  double 
the  first.  Expressing  the  same  fact  in  terms  of  sensitive¬ 
ness  instead  of  exposure,  if  a  plate  is  not  u  quick"  enough 
for  any  given  work,  it  is  rarely  of  use  to  try  a  plate  that 
is  not  at  least  twice  as  sensitive. 

We  may  now  very  well  ask,  What  is  the  nature  of  the 
sensitiveness  to  light  in  this  case  ?  What  change  does 
the  light  produce,  or  what  is  the  difference  between  the 
plate  before  and  after  exposure  ?  In  order  to  answer 
these  questions,  we  must  endeavour  to  get  a  mental  picture 
of  the  plate  and  its  various  parts. 

If  bromide  of  potassium  and  nitrate  of  silver  are 
separately  dissolved  in  water  and  the  solutions  are  mixed, 
a  yellowish  insoluble  substance  will  be  produced,  which 
being  insoluble  will  render  the  liquid  milky  or  opaque,  and 
will  eventually,  if  left  to  itself,  settle  down  to  the  bottom 
of  the  liquid.  The  change  that  takes  place  is  of  a  very 
simple  character,  it  is  merely  that  the  silver  takes  the 
place  of  the  potassium  and  the  potassium  the  place  of 
the  silver.  The  insoluble  substance  is  bromide  of  silver, 
and  the  nitrate  of  potassium  produced  remains  in  solution. 
It  is  this  change  that  takes  place  in  the  preparation  of 
a  gelatino-bromide  emulsion  ;  but  gelatine  is  also  present 
in  the  solution,  and  this  causes  the  bromide  of  silver  to 
be  produced  in  rather  smaller  particles  and  tends  to 
prevent  it  from  settling  down.  To  keep  the  gelatine  liquid 
the  solutions  are  warm,  and  when  the  mixture  is  cooled 
it  sets  to  a  jelly.  This  is  washed  to  get  rid  of  the  nitrate 
of  potassium  and  the  small  excess  of  bromide  of  potassium, 

123 


The  Plate 

treated  as  before  described  to  make  the  silver  salt  more 
sensitive,  and  then  melted  and  spread  upon  the  glass 
plates.  There  is  generally  a  little  iodide  added  as  well 
as  bromide,  and  it  is  possible  that  sometimes  other  things 
may  be  put  in,  but  nothing  further  is  essential.  For 
simplicity's  sake  we  will  disregard  the  iodide  and  other 
possible  additions. 

The  sensitive  plate,  therefore,  consists  of  a  film  of 
gelatine  on  the  glass,  and  throughout  the  gelatine  there 
are  distributed  small  particles  of  bromide  of  silver.  These 
particles  are  very  small,  they  are  not  of  any  particular 
shape,  and  they  make  the  gelatine  film  appear  milky.  It 
is  these  particles  that  are  the  sensitive  compound,  and  they 
exceed  by  very  far  the  sensitiveness  to  light  of  any  other 
known  substance.  In  the  following  consideration  of  the 
plate,  it  is  desirable  to  clearly  imagine  the  tiansparent 
gelatine  film  containing  these  fine  particles  of  biomide 
of  silver  fairly  evenly  distributed  through  it. 

Before  regarding  the  plate  as  a  whole,  we  will  imagine 
that  we  have  only  one  of  these  particles  and  allow  light 
to  act  upon  it  in  repeated  small  doses,  so  that  the  change 
at  every  stage  may  be  noted.  Bromide  of  silver  consists 
of  two  substances,  bromine  and  silver,  combined.  As  is 
usual  when  substances  are  really  combined  as  distin¬ 
guished  from  being  simply  mixed  together,  the  properties 
of  the  individual  substances  are  no  longer  apparent. 
Bromine  is  a  brown  heavy  liquid  of  extremely  unpleasant 
and  choking  odour,  and  silver  is  a  white  malleable  metal. 
Bromide  of  silver  is  not  brown,  it  has  no  odour,  and  it 
is  not  metallic  in  any  of  its  properties.  The  changes  that 
such  a  substance  can  undergo  are  not  very  varied.  Its 
composition  may  be  changed  by  a  separation  or  a  partial 
separation  of  its  constituents,  or  by  adding  some  fresh 
substance  to  it,  or  it  may  be  altered  in  its  nature  without 
any  change  in  its  composition  by  a  variation  of  the 
relationship  that  exists  between  its  various  parts.  The 

124 


The  Plate 

molecules  of  silver  bromide  may,  for  example,  be  grouped 
together  in  a  different  way  or  to  a  different  extent. 
Bearing  in  mind  these  possibilities,  we  will  get  what 
information  we  can  as  to  the  action  of  light  upon  one 
of  the  particles,  which  of  course  must  contain  in  it  a 
very  large  number  of  the  molecules  of  the  compound. 

By  using  a  feeble  light  and  allowing  it  to  act  for  a 
very  short  time  there  is  no  recognisable  change  effected 
in  the  particle.  There  must  be  a  change,  because  if  the 
silver  bromide  were  in  exactly  the  same  condition  as 
before  it  was  subjected  to  the  action  of  light,  then  another 
small  dose  of  light  would  still  leave  it  as  it  was.  But 
this  is  not  so,  for  by  allowing  the  light  to  act  a  little 
longer,  there  is  no  difference  in  the  appearance  of  the 
particle,  it  is  impossible  to  detect  any  separation  of  its 
constituents  or  the  addition  of  anything  to  them,  but  the 
compound  is  now  distinctly  less  stable  than  it  was  before. 
It  is  possible  to  prepare  a  solution  that  has  a  tendency 
to  take  away  the  bromine  and  leave  the  silver,  and  to 
so  adjust  its  power  that  if  the  particle  that  has  been 
acted  on  by  light  is  put  in  it  its  bromine  is  taken  away 
and  only  the  metal  silver  is  left,  while  a  particle  that 
has  not  been  acted  on  by  light  will  remain  in  the  solution 
unchanged.  This  method,  and  a  variation  or  two  of  it, 
is  the  only  known  way  of  discovering  the  difference 
between  an  exposed  and  an  unexposed  particle.  The 
light  has  shaken  the  particle,  or  the  molecules  of  it,  into 
a  less  stable  condition. 

If  a  little  more  light  were  allowed  to  act  on  the  particle 
before  it  was  tested,  it  would  be  found  to  be  still  less 
stable,  or  more  ready  to  give  up  its  bromine  to  the 
solution  that  is  ready  to  take  it.  But  if  the  light  is  allowed 
to  continue  its  action  a  few  stages  further,  the  particle 
begins  to  grow  more  stable  again,  until  jt  reaches  a 
condition  in  which  it  vies  with  the  original  unacted  on 
compound  in  its  power  to  resist  the  loss  of  its  bromine. 

125 


The  Plate 

The  exposed  and  the  unexposed  particles  cannot  now  be 
distinguished  by  this  method;  they  may  be  compared  to 
two  men  racing  round  a  track,  one  of  whom  has  “lapped 
the  other.  To  the  casual  observer  the  two  men  are  level, 
but  if  he  waits  till  the  end  of  the  race  he  will  discover  the 
difference.  By  allowing  more  light  to  act  upon  both 
the  particles  it  will  be  easy  to  tell  that  one  was  a  stage 

in  advance  of  the  other.  ,  rf  ,  ,  ..  ,  , 

After  the  particle  has  by  the  continued  effect  of  light 
upon  it  become  unstable  and  stable  again,  if  light  is 
allowed  to  act  upon  it  still  further  another  change  takes 
place,  it  seems  to  become  unstable  again,  or  it  may  be  that 
the  light  itself  now  separates  the  bromine  from  the  silver, 
as  we  know  it  is  eventually  able  to  do  under  suitable 
circumstances.  Very  little  is  known  about  this  further 
change,  and  it  may  perhaps  be  of  a  different  character 

in  different  cases. 

Now  we  must  return  to  the  gelatine  film  that  contains 
these  particles  distributed  thickly  through  it,  and  bearing 
in  mind  that  the  particles  are  not  all  equally  sensitive, 
endeavour  to  trace  the  effect  of  light  upon  it  as  a  whole. 
It  will  save  circumlocutory  phrases  to  say  at  once,  that 
any  solution  that  is  able  to  take  the  bromine  away  from 
the  silver  bromide  that  has  been  acted  on  by  light  and 
not  from  the  bromide  that  has  not  been  exposed  to  light 
is  called  a  “  developer,”  because  it  develops',  or  brings 
out  or  completes  the  formation  of  the  image.  Although 
the  particles  are  not  all  equally  sensitive,  the  greater 
number  of  them  are  very  nearly  so,  and  there  is  a  small 
proportion  of  less  and  a  small  proportion  of  greater 
sensitiveness.  We  will  suppose  that  light  acts  in  repea  ed 
increments  upon  the  surface  of  the  plate,  and  that  t  e 
effect  of  its  action  is  investigated  by  the  application  ot 
a  developer.  As  the  light  comes  upon  the  surface  it  is 
there  that  its  effect  must  begin,  and  therefore  after  a 
small  exposure  the  more  sensitive  particles  near  the 

126 


The  Plate 

surface  will  be  rendered  amenable  to  the  action  of  the 
developer,  while  the  bulk  of  the  particles  will  be  affected 
only  to  an  extent  that  is  insufficient  to  render  them 
attackable  by  it.  Further  action  of  the  light  will  show 
its  results  deeper  into  the  film,  the  more  sensitive  particles 
lower  down  and  a  much  greater  number  nearer  the 
surface  becoming  unstable.  With  still  more  light  the 
number  of  particles  that  can  be  developed  will  increase, 
always  in  greater  number  near  the  surface  than  below, 
until  the  light  has  acted  sufficiently  to  begin  to  cause 
the  reverse  change  in  the  most  sensitive  particles  that 
lie  near  the  surface.  These  become  stable  again,  and  if 
there  is  an  equivalent  number  of  the  less  sensitive  particles 
remaining,  there  may  be  for  a  certain  time  as  many 
brought  into  the  unstable  condition  as  are  rendered 
stable  again  by  the  excess  of  light  action.  But  eventually 
as  the  light  action  increases  the  number  of  the  particles 
near  the  surface  that  are  unstable  diminishes,  and  this 
effect  gradually  passes  through  the  film.  Therefore, 
considering  the  film  as  a  whole,  the  gradually  increasing 
effect  of  the  light  first  increases  the  number  of  the 
particles  that  are  rendered  unstable  and  then  diminishes 
it.  And  if  we  look  at  the  opacity  of  the  deposit  as  a 
whole  instead  of  the  number  of  particles,  we  see  that 
this  also  will  gradually  grow  first  greater  and  then  less 
as  the  action  of  the  light  increases.  The  proportions  of 
particles  of  various  sensitiveness,  and  possibly  other 
matters,  will  affect  the  relative  rates  at  which  these 
changes  take  place. 

So  far  as  the  effect  in  the  plate  is  concerned,  any 
object  that  is  photographed  is  nothing  more  than  an 
assemblage  of  patches  of  different  shapes  and  different 
brightnesses.  In  the  image  that  falls  upon  the  plate 
in  the  camera  there  is  the  same  disposition  of  parts  and 
the  same  range  of  brightness,  if  it  is  truly  produced. 
The  exposure  of  the  plate  is,  of  course,  of  the  same 

127 


The  Plate 

duration  for  every  part  of  the  image,  and  it  is  the 
varying  brightness  of  the  different  parts  that  causes  a 
different  amount  of  light  action  on  the  different  parts  o 
the  plate  and  so  leads  to  the  production  of  the  image  in 
the  negative.  What  is  wanted  is  that  the  amount  of  change 
produced  in  any  part  of  the  plate,  which  may  be  regarded 
in  general  terms  as  the  number  of  particles  changed 
into  the  unstable  condition,  shall  vary  in  accordance 
with  the  brightness  of  the  image  that  falls  upon  that  part. 

We  are  now  in  a  position  to  understand  the  effect  o 
too  little  or  too  much  exposure,  and  we  will  begin,  as  in 
the  previous  cases,  with  a  small  exposure  to  trace  the 
effect  as  it  is  prolonged  to  a  very  much  greater  extent 
than  it  is  ever  likely  to  be  in  practice.  It  is  the  time  of 
the  exposure  that  decides  the  extent  of  the  action.  With 
a  very  short  time,  the  brightest  parts  of  the  image  will 
have  produced  a  considerable  change  and  a  satisfactorily 
proportional  one,  but  the  parts  of  medium  brightness 
will  have  fully  affected  only  the  few  unduly  sensitive 
particles,  and  the  duller  parts  of  the  image  will  not  have 
been  able  to  do  even  this  much.  The  developed  plate 
will  show  the  high  lights  well,  but  the  middle  tones  will 
be  feebly  represented,  and  the  detail  in  the  darker  parts 
of  the  subject  will  not  be  shown  at  all— where  they  should 
be  will  be  a  blank.  A  print  from  such  a  negative  would 
show  the  dark  parts  of  the  subject  as  uniform  black 
patches,  and  the  detail  in  the  middle  tones  would  be 
lacking  in  contrast-"  flat."  By  prolonging  the  exposure 
somewhat,  the  brightest  parts  of  the  image  produce 
correspondingly  more  effect,  the  middle  tones  take  their 
proper  place,  but  the  darkest  parts  still  have  not  had 
time  enough  to  act  on  more  than  the  small  proportion 
of  the  unduly  sensitive  particles,  and  although  the 
detail  here  is  visible,  it  is  weak  and  flat.  This  is  under¬ 
exposure,  and  in  the  first  case  the  under-exposure  is 

exaggerated. 


128 


The  Plate 

By  giving  a  longer  exposure  the  brightest  parts  and 
the  middle  tones  continue  to  give  an  increased  effect 
and  the  darkest  parts  now  take  their  proper  place.  This 
is  correct  exposure,  and  the  print  from  the  negative,  if 
both  are  properly  made,  will  give  the  most  true  repre¬ 
sentation  of  the  object  that  is  possible  under  the  con¬ 
ditions  that  hold.  By  prolonging  the  exposure  all  parts 
will  continue  to  increase  the  action  that  they  effect,  and 
the  negative  will  still  give  a  good  print,  but  being  denser 
all  over,  it  will  take  longer  to  get  the  print.  There  is 
thus  with  a  good  plate,  that  is,  one  that  has  a  liberal 
supply  of  emulsion,  and  an  average  subject,  always  a 
certain  range  of  exposures  that  will  give  a  good  and  . 
true  negative.  There  is  a  minimum  and  a  maximum 
exposure  between  which  the  errors  resulting  from  both 
under  and  over  exposure  are  avoided.  What  this  range 
is,  clearly  depends  on  both  the  plate  and  the  subject. 

If  now  the  exposure  is  prolonged  still  further,  the 
brightest  parts  of  the  image  will  have  acted  on  all  the 
particles  in  that  part  of  the  plate  that  they  fall  upon,  or, 
if  the  film  of  emulsion  is  too  thick  for  this  to  take  place, 
the  particles  in  the  surface  of  the  film  will  have  been 
so  much  affected  that  they  will  be  beginning,  the  most 
sensitive  of  them,  to  assume  the  more  stable  condition. 
In  either  case,  the  effect  of  the  brightest  parts  of  the 
image  will  cease  to  increase  with  the  prolonged  exposure 
at  the  due  rate,  and  therefore  the  contrast  here  will 
diminish,  and  in  the  print  the  detail  in  the  high  lights 
will  be  flat  and  weak.  The  other  parts  still  retain  their 
proper  proportional  relationship  to  each  other.  By 
still  increasing  the  exposure  this  flatness  extends  to  the 
less  bright  parts  and  eventually  perhaps  to  the  whole 
image.  With  a  still  further  prolongation  of  the  exposure, 
the  brightest  parts  will  change  an  increasing  number  of 
particles  into  the  second  stable  condition,  and  so  will 
produce  a  diminishing  instead  of  an  increasing  effect, 

129  I 


The  Plate 

and  the  highest  lights  may  thus  appear  in  the  print  as  if 
they  were  actually  less  bright  than  those  parts  of  the 
image  which  are  really  inferior  to  them  in  brightness. 
This  is  “  reversal,”  and  is  the  natural  effect  of  great  over 
exposure.  It  is  possible  to  push  on  this  reversal  until 
it  is  complete,  and  instead  of  a  negative  where  the  highest 
light  is  represented  by  the  densest  deposit,  the  result  is 
a  positive  with  the  densest  deposit  standing  for  the 
darkest  part  of  the  object.  The  getting  of  such  a  result 
satisfactorily  needs  special  precautions,  and  as  it  is  always 
uncertain,  it  is  not  a  method  of  work  that  is  to  be  re¬ 
commended.  Sometimes  it  would  be  economical  and 
time  saving,  but  it  has  been  tried  and  experience  always 
condemns  it. 


130 


CHAPTER  VIII 

THE  EXPOSURE 


A  CAMERA  is  literally  a  chamber ,  and  a  camera  obscura  is 
a  dark  chamber.  The  words  as  applied  in  photography 
are  not  well  chosen,  for  there  need  be  nothing  specially 
chamber-like  in  a  camera,  and  it  is  dark  only  when  it  is 
not  in  use,  for  it  is  light  that  does  the  work.  A  small 
chamber  is  more  or  less  of  a  box,  and  it  is  not  uncommon 
to  speak  of  a  “  box  ”  camera  to  distinguish  it  from  a  bellows 
collapsible  camera.  In  America  a  camera  is  often  called  a 
“  box.” 

This  chamber  or  box  idea  is  misleading  to  those  who 
do  not  understand  the  subject.  It  was  rather  by  accident 
than  otherwise  that  some  of  the  earliest  photographic 
cameras  were  box-like  in  outline,  and  it  is  not  desirable  to 
emphasise  this  similarity  because  it  draws  attention  from 
the  actual  nature  and  use  of  the  instrument.  A  camera 
should  be  regarded  rather  as  a  stand  or  support,  because 
its  true  function  is  to  hold  the  lens  and  the  sensitive  plate 
in  that  relationship  to  each  other  that  the  photographer 
desires.  The  tubes  of  a  telescope  play  the  same  part  with 
regard  to  that  instrument ;  they  support  the  object  glass  at 
one  end  and  the  eyepiece,  with  which  the  image  is  viewed, 
at  the  other.  In  the  case  of  the  microscope  stand,  the 
object  to  be  viewed  also  has  to  be  supported,  and  this  leads 
to  a  measure  of  complication.  But  so  far  as  the  optical 
part  of  the  apparatus  is  concerned,  the  only  adjustment 
wanted  in  both  the  telescope  and  the  microscope  in  their 
simple  forms,  is  the  power  to  alter  the  distance  between 

Hi 


The  Exposure 

the  object-glass  and  the  eyepiece  so  that  the  image  may  be 
clearly  seen,  or,  as  we  say,  that  it  may  be  focussed.  The 
earliest  cameras  also  were  provided  with  only  this  adjust¬ 
ment,  as  photography  was  such  a  novelty  that  the  mere 
getting  of  an  image  of  a  passable  quality  satisfied  the 
worker. 

In  Fig.  20  three  old  cameras  are  shown  as  they  were 
made  during  the  first  ten  years  of  photographic  practice 
immediately  following  the  introduction  of  the  Daguerreo- 


Fig.  20. — Early  types  of  cameras. 


type  and  Talbotype.  The  first  drawing  shows  one  for 
taking  photographs  on  paper.  It  has  a  meniscus  lens 
carried  in  a  tube  that  slides  in  its  fitting  for  the  purpose  of 
focussing.  The  next  is  by  Voigtliinder  and  is  obviously 
designed  on  the  simple  telescopic  plan.  The  body  of  the 
instrument  is  made  of  round  brass  tubes,  some  conical 
to  give  the  increase  in  size  necessary  to  accommodate  the 
plate.  The  shorter  conical  portion  that  extends  behind  the 
plate  carries  a  small  lens  through  which  the  observer  can 
see  when  he  has  got  the  image  in  good  focus.  The  third 
camera  has  a  very  large  portrait  lens,  desirable  at  that 
time  in  order  that  the  exposure  might  be  shortened,  and 

132 


The  Exposure 

it  extends  only  two  or  three  inches  behind  the  lens  and  is 
otherwise  small,  because  the  defining  power  of  a  lens  of 
such  a  large  diameter  was  limited  to  a  very  small  area. 
In  all  these  cases  there  is  the  simplicity  of  the  ordinary 
telescope,  the  variation  being  merely  that  the  part  that  is 
to  take  the  plate  or  the  sensitive  paper  is  made  larger  in 
order  to  properly  accommodate  it. 

In  a  small  guide  to  the  practice  of  photography,  pub¬ 
lished  in  1854,  we  read  that  the  sides  of  the  camera  may 
with  advantage  consist  of  a  “  bag  of  cloth  or  caoutchouc  ” 
stretched  between  the  back  and  the  front.  We  further 
read  that  “  instead  of,  and  even  in  preference  to,  the  cloth 
or  caoutchouc  bag  camera,  one  of  pasteboard  may  also 
be  used,  made  in  the  same  shape  as  the  bellows  of  an 
accordion."  By  i860,  or  about  then,  the  bellows  were 
sometimes  made  tapered  towards  the  lens,  so  gaining  in 
lightness  and  compactness.  Thus  we  see  how  varied  was 
the  “box"  part  of  the  camera — it  was  made  of  all  shapes 
and  all  materials,  its  only  duty  being  to  keep  out  extraneous 
light,  so  that  the  image  produced  by  the  lens  might  be 
received  by  the  plate  in  its  full  brightness  and  purity. 

Let  us  endeavour  to  see  what  the  modern  photographer 
wants  his  camera  to  do,  bearing  in  mind  that  it  is  very 
desirable,  indeed  we  may  say  necessary,  that  the  lens  shall 
produce  a  greater  extent  of  image  than  is  sufficient  to 
cover  the  plate.  In  order  to  cause  the  image  to  conform 
to  the  rules  of  plane  perspective,  it  is  necessary  that  the 
plate  shall  be  vertical  when  photographing  fixed  objects 
or  those  that  are  to  be  represented  in  their  natural 
positions.  For  if  the  plate  is  not  vertical,  the  scale  of  the 
image,  that  is  its  dimensions  compared  with  the  dimensions 
of  the  object,  after  making  due  allowance  for  the  different 
distances  of  the  various  parts,  will  decrease  in  the  direction 
of  that  part  of  the  plate  that  is  tipped  forwards  nearer  to 
the  lens.  So  far  as  the  scale  is  reduced,  vertical  and 
therefore  parallel,  lines  in  the  object  will  be  represented  as 

133 


The  Exposure 

if  they  inclined  towards  each  other.  This  may  be  seen  in 
beginners'  attempts  when  they  point  their  camera  upwards 
to  photograph  a  high  object  and  make  no  counteracting 
adjustment;  the  building  is  represented  as  tapering  up¬ 
wards  instead  of  with  its  sides  parallel. 

If  then  we  start  with  the  plate  vertical  and  the  lens 
exactly  opposite  the  centre  of  it,  the  image  of  the  horizon 
will  fall  across  the  very  middle  of  the  plate.  It  is  very 
rarely  that  it  is  wanted  in  this  position.  If  we  are  photo¬ 
graphing  a  building  that  is  chiefly  above  the  horizon  we  do 
not  want  the  photograph  to  show  the  house  entirely  in  the 
upper  half  of  the  picture  with  the  lower  half  occupied  by 
an  expanse  of  uninteresting  foreground  ;  and  if  the  aim  of 
the  photographer  is  to  get  a  beautiful  expanse  of  level 
country,  he  does  not  want  an  exactly  equal  expanse  of  sky 
above  it.  Truly  he  might  cut  off  from  the  print  what 
he  does  not  want,  but  that  would  mean  wasting  nearly 
half  of  the  plate  in  each  case,  besides  having  to  carry  about 
a  camera  of  nearly  twice  the  dimensions  really  necessary 

for  the  work.  . 

The  most  obvious  method  of  meeting  this  difficulty 

would  be  to  provide  for  the  movement  of  the  plate  up 
and  down,  so  that  by  adjusting  its  position  the  photo¬ 
grapher  should  be  able  to  place  it  where  it  would  receive 
exactly  that  part  of  the  image  produced  by  the  lens  that 
he  wished  for.  But  this  would  mean  a  large  and  corre¬ 
spondingly  heavy  camera,  and  as  we  know  that  it  is 
simply  a  question  of  the  relative  positions  of  the  lens  and 
the  plate,  moving  the  lens  in  the  opposite  direction  to 
the  required  movement  of  the  plate  would  have  the  same 
result.  The  horizon  is  level  with  the  lens,  so  that  by 
raising  or  lowering  it  the  position  of  the  horizon  on  the  plate 
can  be  regulated.  The  drawback  to  this  substituted  method 
of  adjustment  is  that  the  lens  is  the  point  of  view,  and  shifting 
the  point  of  view  an  inch  or  two,  while  negligible  under  ordi¬ 
nary  circumstances,  may  be  undesirable  with  near  objects. 

134 


The  Exposure 

But  if  the  object  is  very  high  and  comparatively  near, 
the  camera  may  have  to  be  pointed  upwards  in  order  to 
get  the  image  on  the  plate  at  all.  For  the  sake  of  correct 
“drawing"  it  is  necessary  next  to  restore  the  plate  to 
the  vertical  position,  and  this  requires  that  the  part  of 
the  camera  that  carries  the  plate  shall  be  made  to  swing 
like  a  toilet  looking-glass.  Hence  the  need  for  a  “  swing 
back." 

The  image  given  by  the  lens  may  be  too  small,  so 
that  the  plate  receives  the 'representations  of  things  around 
it  that  are  not  wanted.  A  lens  of  greater  focal  length 
will  give  the  image  on  a  larger  scale,  but  then  to  focus 
it  the  plate  must  be  brought  to  a  greater  distance  from 
the  lens— there  must  therefore  be  ample  focussing  adjust¬ 
ment,  or  range  of  length,  in  the  camera.  For  a  similar 
reason,  but  in  the  opposite  direction,  it  is  necessary  to 
be  able  to  push  the  camera  “back,"  with  the  plate  it 
carries,  near  up  to  the  lens.  For  some  pictures  the  longer 
dimension  of  the  plate  must  be  perpendicular,  for  others 
horizontal ;  if  therefore  the  camera  cannot  be  turned  bodily 
on  to  its  side,  and  with  any  but  the  smallest  cameras 
this  is  not  desirable,  the  back  that  carries  the  plate 
must  be  made  square  and  “reversible,"  that  it  may  be 
attached  to  the  camera  in  either  position. 

All  camera  adjustments  are  of  the  type  of  those 
mentioned  and  refer  merely  to  the  relative  positions  of 
the  lens  and  plate,  and  therefore  have  no  direct  influ¬ 
ence  on  the  quality  of  the  photographs  produced.  A 
simple  box  of  cardboard  that  would  hold  the  plate  and 
lens  firmly  in  position  would  give  as  good  a  result  as 
the  most  costly  camera,  but  it  would  be  inconvenient, 
because  in  the  next  photograph  a  different  arrangement 
might  be  necessary  and  with  a  non-adjustable  apparatus 
it  would  be  impossible  to  make  the  change.  From  this 
point  of  view  these  adjustments  are  necessary,  and  the 
manner  in  which  they  are  provided  for  at  the  piesent 

135 


The  Exposure 

time  is  indicated  in  Figs.  21  and  22,  of  two  cameras 
by  Messrs.  Watson  &  Sons.  The  “Acme”  with  its  taper- 


Fig.  21. — Watson’s  “Acme”  camera. 


ing  bellows  folds  up  into  a  small  space  and  is  light,  while 
the  “  Premier"  with  its  full  size  and  fixed  front  is  heavier, 


but  to  be  preferred  in  some  of  its  details.  Excluding 
hand  cameras,  which  will  be  subsequently  considered, 

136 


The  Exposure 

these  are  the  two  standard  types  for  work  out-of-doors, 
with  of  course  many  variations  in  details  according  to 
the  individual  maker. 

For  photography  indoors  the  requirements  are  differ¬ 
ent.  The  professional  portrait  photographer  does  not 
want  portability  beyond  the  power  to  wheel  his  stand  and 
camera  about  his  studio.  The  necessary  range  for  focuss¬ 
ing  is  more  limited,  he  generally  uses  a  larger  and  much 
heavier  lens,  and  he  wants  to  quickly  insert  the  plate  after 
focussing  so  that  he  may  not  inconvenience  the  model. 
His  apparatus  may  be  heavy,  but  it  must  be  such  that  he 
can  quickly  and  easily  manipulate  it.  The  trade  photo¬ 
grapher  often  needs  cameras  that  will  take  very  large  plates 
for  copying  work,  and  here  again  details  are  varied  to 
suit  the  special  requirements  of  the  case.  It  is  obvious 
that  the  most  convenient  apparatus  for  a  plate  four  or 
five  inches  long  may  not  be  suitable  for  the  manipulation 
of  a  heavy  plate  two  or  three  feet  long.  But  the  general 
principles  remain  the  same  whether  the  camera  is  as 
small  as  the  hand,  or  is  so  large  that  the  operator  can 
get  right  inside  it  in  order  to  focus  and  adjust  the  image. 

Work  indoors  is  generally  more  under  control  than 
work  out-of-doors,  and  therefore  is  preferable  when  the 
object  to  be  photographed  is  movable  and  other  circum¬ 
stances  permit  it.  It  is  not  only  that  adverse  climatic 
conditions,  such  as  wind  and  rain,  are  excluded,  but  that 
the  light  that  illuminates  the  object  may  be  adjusted  to 
the  best  advantage,  by  means  of  blinds  or  shutters  to 
exclude  or  modify  it  as  desired.  An  apartment  devoted 
to  this  kind  of  work  is  called  a  u  studio." 

A  studio  for  trade  work,  that  is  copying,  enlarging, 
and  such  photography  as  is  required  for  the  making  of 
printing  blocks  and  plates,  would  perhaps  be  better  de¬ 
scribed  as  a  photographic  workshop,  for  there  is  nothing 
to  consider  in  its  arrangements  but  the  efficiency  of  the 
work,  and  therefore  it  is  full  of  the  necessary  apparatus. 

137 


The  Exposure 

The  earliest  portrait  studios  were  arranged  on  somewhat 
similar  lines  :  everything  was  subservient  to  the  photo¬ 
graphy,  and  the  person  to  be  photographed  was  treated 
very  much  as  if  he  were  an  inanimate  object  to  be  oper¬ 
ated  upon.  Indeed  the  photographer  himself  was  called 
an  operator,  and  there  is  no  wonder,  therefore,  that  when 
the  novelty  of  having  a  picture  of  one’s  self  produced 
automatically  wore  off,  and  people  went  to  the  photo¬ 
graphers  to  please  their  friends  rather  than  themselves, 
that  they  should  feel  as  if  they  were  going  to  have  some¬ 
thing  done  to  them,  and  compare  their  visits  to  the  photo¬ 
grapher  with  their  visits  to  the  dentist.  In  both  cases 
there  was  a  special  kind  of  chair  and  an  arrangement  for 
steadying  or  holding  the  head,  and  the  light  was  specially 
managed  to  illuminate  the  sitter.  The  similarity  was 
much  greater  than  those  of  the  present  generation  can 
realise.  The  studio  was  too  often  lumbered  up  with  a 
multiplicity  of  paraphernalia  —  backgrounds,  head-rests, 
screens,  peculiar  uncomfortable  and  almost  indescribable 
pieces  of  furniture,  some  of  which  could  be  changed  by 
a  little  alteration  into  something  of  quite  a  different  kind. 
There  might  be  things  made  to  imitate  a  fence,  a  stile, 
or  a  balcony,  mats  to  imitate  grass,  an  imitation  boat,  or 
rather  half  a  boat,  because  as  the  other  side  would  not  be 
seen  there  was  no  need  to  have  it.  The  idea  was  to  be 
able  to  imitate  in  the  studio  any  scene  either  indoors  or 
out-of-doors.  Now  imitations  of  outdoor  scenes  in  a  room 
are  always  incongruous.  The  light  that  falls  upon  a 
person  in  a  building  is  restricted  as  to  its  direction,  and 
this  causes  a  greater  difference  in  brightness  between  the 
side  next  to  the  light  and  the  side  away  from  it,  and,  if 
well  managed,  a  better  appearance  of  roundness  and 
modelling  than  is  often  possible  in  the  open  air  where 
the  light  comes  more  equally  from  all  directions.  An 
indoor  lighting  of  the  figure  with  outdoor  scenery  must 
always  be  false. 


138 


A  View  in  Mr.  Hoppe’s  Studio 


E.  O.  Hoppe. 


The  Exposure 

The  early  portrait  studios  had  to  have  a  great  deal  of 
glass,  because  the  necessary  period  of  the  exposure  of 
the  plate  was  long  enough  to  make  it  desirable  to  do  every¬ 
thing  that  would  tend  to  shorten  it.  They  were  made  of 
many  shapes.  Some  were  tunnel-like,  with  an  expansion 
at  one  end  to  accommodate  the  sitter,  others  were  large 
apartments  with  huge  skylights,  but  the  same  idea  was 
manifest  in  all,  namely  to  get  effective  lighting  and  to  get 
as  much  light  as  consistent  with  the  result  desired  that 

the  exposure  might  be  shortened. 

As  the  manufacture  of  more  sensitive  plates  became 
possible,  as  the  novelty  of  being  photographed  wore  off, 
and  as  portrait  photographers  grew  to  understand  their 
business  better,  they  began  to  think  more  of  those  who 
patronised  them,  instead  of  being  absorbed  in  the  simple 
manipulative  part  of  the  work.  The  sham  properties 
gradually  disappeared,  and  now  in  the  best  studios  all  the 
furniture  is  of  approved  patterns,  solid,  and  good,  and  all 
the  appointments  are  such  as  tend  to  please  the  eye  and 
conduce  to  comfort.  A  good  modern  studio  is  comparable 
to  a  drawing-room  rather  than  to  a  workshop,  and  the 
skilful  portraitist  now  finds  no  need  to  ask  his  sitter  to 
assume  any  particular  expression.  The  air  of  artificiality 
has  given  place  to  an  atmosphere  of  truth.  By  the  kind¬ 
ness  of  Mr.  E.  O.  Hoppe  we  are  able  to  give  a  photograph 
of  the  studio  that  he  has  recently  opened  at  59  Baker 
Street,  London.  Those  who  visit  Mr.  Hopp<§  for  the  first 
time  are  loath  to  believe  that  this  is  really  his  studio,  for 
it  is  nothing  more  than  an  ordinary  large  room  on  the 
first  floor  of  the  house,  with  three  large  ordinary  windows 
that  face  the  street.  Nothing  of  the  character  of  photo¬ 
graphic  apparatus  is  evident,  and  it  is  not  until  the  visitor 
has  settled  himself  comfortably  and  naturally  and  is 
obviously  at  his  ease,  that  the  camera  is  brought  in  and 
the  exposures  made.  Mr.  Hopp6  has  favoured  11s  also 

with  an  example  of  his  work. 

139 


The  Exposure 

But  whether  it  is  a  person  or  an  inanimate  object  that 
is  being  photographed,  the  actual  photography  is  much 
the  same.  The  image  formed  by  the  lens  has  to  be 
allowed  to  fall  upon  the  plate  for  a  suitable  time.  There 
is  a  little  licence,  a  little  difference  between  the  minimum 
and  the  maximum  for  the  best  result,  but  if  these  limits 
are  overstepped  in  either  direction  the  result  suffers.  This 
range  of  exposure  is  considerably  longer  than  some 
workers  make  believe.  We  may  sometimes  hear  such 
an  expression  as,  “  I  told  him  to  give  twenty  seconds 
and  he  gave  twenty-two,  and  the  exposure  was  just  those 
two  seconds  too  long."  Such  a  statement  as  this  is  not 
true,  because  a  difference  in  the  exposure  time  of  one 
in  ten  is  not  detectable  to  the  unassisted  eye  either  in  the 
negative  or  the  print.  If  the  twenty-two  seconds  was 
obviously  too  long  as  exposure,  it  is  fairly  certain  that 
eleven  seconds  would  have  been  better,  and  probably  a 
difference  midway  between  these  would  have  been  the 
very  least  alteration  of  the  time  that  would  have  been 
recognisable  in  the  result. 

The  time  of  exposure  required  depends  upon  the  light, 
the  sensitiveness  of  the  plate,  the  object,  and  the  lens 
aperture,  and  each  one  of  these  factors  must  enter  into 
the  calculation.  Some  time  ago  one  might  happen  to 
meet  a  photographer  of  the  old  school  who  despised 
entering  into  such  details  as  these.  If  he  had  an  idea  of 
the  sensitiveness  of  the  plate,  and  he  was  not  always  very 
particular  even  about  that,  then  he  would  say  that  the 
best  way  to  estimate  the  exposure  necessary  was  just  to 
look  at  the  image  on  the  ground  glass  and  judge  from  its 
brightness  as  to  how  long  should  be  given.  The  author 
once  tested  an  old  Daguerreotypist  who  professed  to  work 
in  this  way  by  asking  him  as  to  what  exposure  he  would 
advise,  and  then  immediately  changing  the  lens  aperture 
to  a  definite  extent  and  asking  him  again.  He  had  no 
idea  of  the  object  of  the  inquiry,  so  that  his  two  estimates 

140 


The  Exposure 

were  quite  genuine  and  independent  of  each  other.  As 
the  only  change  was  in  the  lens  aperture  the  difference 
between  the  two  exposures  needed  was  perfectly  definite, 
but  the  two  estimates  of  the  man  of  experience  bore  a 
very  different  relationship  to  each  other.  The  fact  is, 
that  it  is  impossible  for  anyone  to  estimate  the  com¬ 
parative  brightness  of  two  images  even  when  they  are 
presented  to  him  the  one  immediately  after  the  other, 
much  less  when  there  is  a  considerable  interval  between 
the  inspection  of  them.  And  if  it  were  possible  to  cor¬ 
rectly  estimate  the  brightnesses  of  the  two  images,  that 
would  not  be  sufficient,  because  the  exposure  necessary 
is  not  always  proportional  to  the  visual  brightness,  as 
we  shall  shortly  see. 

But  it  must  not  be  thought  that  photographers  of 
the  kind  mentioned  were  untruthful  or  did  not  give 
their  advice  in  good  faith.  They  had  not  the  facilities 
that  modern  photographers  have  for  estimating  the 
duration  of  the  exposure  that  is  necessary.  Thus  they 
had  to  depend  upon  their  own  unaided  experience,  and 
this  was  a  much  more  complex  matter  than  they  con¬ 
sidered  it  to  be.  There  can  be  no  doubt  that  although 
they  thought  that  their  estimates  were  based  upon  a 
simple  observation,  they  unconsciously  took  many  other 
circumstances  into  account.  It  is  very  difficult  to 
analyse  our  mental  processes.  Ask  anyone  with  his  eyes 
shut  to  touch  the  tip  of  his  nose  with  the  end  of  any 

of  his  fingers  that  you  may  mention  and  he  will  pro¬ 

bably  do  it  at  once  and  with  precision.  Now  give  him 
a  lead  pencil  or  pen  stick,  he  sees  the  length  of  it,  and 
ask  him  to  shut  his  eyes  and  touch  his  nose  with  the 
end  of  it.  He  will  probably  do  it  with  almost  the  same 

degree  of  precision  as  with  his  finger  end.  But  now 

for  the  straight  stick  substitute  a  curved  one  such  as  a 
clay  tobacco  pipe.  He  sees  the  curve  and  he  sees  the 
length  of  it,  but  he  will  probably  fail  altogethei.  It 

I4I 


The  Exposure 

would  be  difficult  to  say  what  circumstances  were  taken 
into  account  in  the  first  parts  of  the  experiment,  but  it 
is  certain  that  the  introduction  of  a  new  element,  the 
curvature,  renders  the  performance  impossible.  How¬ 
ever  expert  one  may  become  in  estimating  exposure  by 
a  simple  inspection  of  the  image  on  the  ground  glass, 
there  always  remains  the  uncertain  element  of  the 
variable  proportion  between  the  visual  brightness  and 
the  photographic  power  of  the  light. 

So  far  as  the  light  is  concerned,  it  is  desirable  to 
estimate  its  photographic  power  irrespective  of  the 
object.  It  is  not  the  light  that  comes  from  the  object 
to  the  plate  that  is  to  be  measured,  but  the  light  that 
falls  upon  the  object.  If  the  exposure  were  to  be 
regulated  by  the  intensity  of  the  light  coming  from  the 
object,  then  a  dark  oak  carving  and  a  white  plaster 
model  of  it  would  give  very  similar  results,  because  the 
darkness  of  the  oak  would  be  compensated  for  by  the 
proportionally  increased  exposure.  This  obviously  would 
be  incorrect,  for  we  want  the  dark  object  to  appear 
dark,  and  the  white  object  white  in  the  photograph, 
and  not  both  to  appear  as  if  there  was  no  difference 
between  them. 

It  is  possible  to  estimate  the  general  intensity  of  the 
light  by  remembering  that  the  best  light  at  noon  in  mid¬ 
winter  has  about  one-fifth  the  value  of  the  best  light  at 
noon  in  mid-summer,  and  tables  are  available  to  show 
in  what  proportion  the  light  falls  off  for  each  hour  of 
the  day.  But  the  clouds  interfere  to  a  very  conspicuous 
degree  with  the  light  intensity.  There  is  a  ceitain 
measure  of  darkness  that  would  cause  anyone  to  give 
up  all  idea  of  photographing — such  fogs,  for  example, 
that  make  it  necessary  to  resort  to  artificial  illumination 
as  if  it  were  night.  But  considering  only  those  degrees 
of  light  that  would  not  obviously  stop  work  by  daylight, 
we  may  make  five  divisions,  and  double  the  exposure 

142 


The  Exposure 

up  to  five  times  according  to  whether  the  sun  is  obscured, 
the  sky  is  cloudy,  dull,  or  very  dull.  When  working  by 
this  method  the  character  of  the  object  is  of  vital  im¬ 
portance,  for  by  simply  going  into  a  shady  place,  as  in 
a  wood,  the  exposure  as  compared  with  that  necessary 
in  the  open  may  need  to  be  a  hundred  times  as  long, 
without  going  to  an  absurd  extreme.  Tables  have  been 
drawn  up  giving  the  exposures  needed  founded  on  these 
circumstances,  and  sometimes  these  tables  have  been 
put  into  the  form  of  a  slide  rule  or  of  revolving  cylinders. 
Whatever  form  they  assume  the  principle  of  their  con¬ 
struction  is  the  same,  though  the  slide  rule  or  the  re¬ 
volving  cylinders  may  be  a  little  more  convenient  or 
compact.  Such  a  system  of  estimating  the  required 
exposure  is  better  than  no  system  at  all. 

The  best  way  to  estimate  the  light  is  to  measure  it, 
not  visually  but  photographically.  Ordinary  white  light, 
as  we  have  already  seen,  is  not  a  simple  single  move¬ 
ment,  but  is  a  regularly  graduated  series  of  movements, 
and  it  is  only  a  comparatively  small  part  of  these  that 
affect  the  eye  so  as  to  cause  the  sensation  of  light.  It 
is  another  part  of  them  that  is  of  general  photographic 
efficiency,  and,  speaking  generally,  all  different  things 
that  are  affected  by  light  are  changed  by  more  or  less 
different  constituents  of  the  mixture  of  radiations  that 
we  commonly  speak  of  as  light.  The  actinic  power  of 
light,  at  least  in  a  practical  sense,  does  not  depend 
merely  upon  the  brilliancy  of  the  light  to  the  eyes,  as 
used  in  earlier  times  to  be  taken  for  granted.  Therefore 
the  only  way  to  exactly  measure  the  comparative  power 
of  lights  to  affect  any  given  substance,  is  to  use  that  sub¬ 
stance  itself  in  making  the  test.  For  gelatino-bromide 
plates,  the  same  kind  of  plate  must  be  subjected  to  the 
same  treatment  as  to  development  in  making  the  test  as 
the  plates  will  be  for  the  actual  exposures  on  the  objects 
to  be  photographed.  But  such  a  method  of  testing  the 

143 


The  Exposure 

light  would  be  tedious  and  often  impracticable,  and  as 
there  is  always  a  certain  amount  of  variation  permissible 
in  the  duration  of  the  exposure,  this  only  exact  method 
may  be  advantageously  replaced  by  a  rapid  and  practic¬ 
able  method  that  is  almost  always  good  enough.  Paper 
is  coated  with  a  bromide  of  silver  emulsion  and  so 
treated  or  “sensitised'’  that  when  exposed  to  light  it 
rapidly  darkens  without  any  need  for  development.  Thus 
the  same  compound,  silver  bromide,  is  used  as  in  the 
actual  plates,  but  its  treatment  is  varied  in  order  to 
make  it  a  more  practical  test.  This  is  a  very  muc 
superior  method  to  any  process  of  estimating  brightness 

by  the  eye. 

The  modern  actinometer  has  a  piece  of  such  sensitive 
paper  under  a  screen  with  a  small  hole  in  it,  and  when 
exposed  to  the  light  to  be  tested,  the  unprotected  portion 
of  the  paper  darkens,  and  when  its  darkness  matches  that 
of  a  painted  tint  by  the  side  of  the  hole  in  the  screen  the 
test  is  complete.  For  each  test  a  fresh  surface  of  the 
sensitive  paper  is  drawn  under  the  opening,  or  the  screen 
is  rotated  so  as  to  expose  a  fresh  portion.  The  exposure 
necessary  is  directly  proportional  to  the  time  required  for 
the  paper  to  darken  to  the  standard  tint.  If  the  paper  takes 
twice  as  long  to  darken  as  it  did  on  a  previous  occasion, 
then,  under  exactly!  similar  circumstances  otherwise,  the 
exposure  of  the  plate  must  be  twice  as  long  as  on  the  first 
occasion.  Most  of  the  actinometers  have  movable  scales 
attached  to  the  cases  to  facilitate  the  calculation  of  the  re¬ 
quired  exposure  for  plates  of  different  sensitiveness  and 
with  various  lens  apertures,  and  then  the  apparatus  is 
called  an  “  exposure  meter.”  On  a  fine  summer’s  day  out- 
of-doors  in  the  shade  the  time  taken  for  the  paper  to 
darken  will  be  from  two  to  four  seconds,  so  that  the  test 
of  the  light  is  rapidly  made.  For  indoors,  when  the  test 
might  be  tedious,  a  lighter  comparison  tint  is  used  to  save 

time. 


144 


The  Exposure 

The  effect  of  the  aperture  of  the  lens  on  the  exposure 
we  need  not  trouble  further  about,  because  it  is  clear  that 
the  larger  the  opening  the  more  light  is  admitted  and 
therefore  the  shorter  the  exposure  required.  The  lens 
diaphragms  are  marked  in  such  a  manner  that  the  next 
larger  or  smaller,  doubles  or  halves  the  area  of  the  aper¬ 
ture  and  therefore  needs  a  half  or  double  duration  of  the 
exposure. 

It  might  be  supposed  that  theoretically  the  character  of 
the  object  to  be  photographed  should  not  be  taken  into 
account  in  determining  what  the  duration  of  the  exposure 
shall  be,  because  we  want  a  dark  object  to  produce  but 
little  effect  on  the  plate  that  it  may  appear  dark  in  the 
print,  while  a  light  or  white  object  should  produce  an 
increased  effect  corresponding  to  its  brightness  that  it  may 
appear  to  be  white.  This  is  true  in  principle  when  light 
and  dark  objects  are  present  together  in  the  view  or 
collection  of  things  that  is  to  be  photographed,  and  indeed 
it  is  obvious  that  it  would  be  impossible  to  adjust  the 
exposure  to  the  various  parts  of  the  object  even  if  it  were 
desired  to  do  so.  Sometimes  this  is  attempted  in  a  crude 
way  by,  for  example,  shading  the  upper  part  of  the  lens  so 
that  the  light  that  represents  the  sky  in  the  picture  may  be 
reduced.  But  such  procedure  is  always  uncertain  in  its 
results  and  rarely  satisfactory.  When,  however,  we  are 
dealing  with  an  object  that  is  altogether  dark  or  altogether 
light,  it  is  practically  advantageous  to  adjust  the  exposure 
a  little,  giving  say  twice  the  indicated  exposure  in  the  first 
case  and  half  in  the  second.  This  has  the  effect  of  making 
the  detail  a  little  more  conspicuous  in  the  photograph,  and 
actually  makes  the  representation  more  true  to  the  appear¬ 
ance  of  the  object,  because  when  we  look  at  an  object  that 
is  altogether  dark,  our  eyes  naturally  adjust  themselves  to 
the  darkness  by  the  expansion  of  the  iris,  and  the  dark 
object  does  not  appear  quite  so  dark  as  it  would  if  it  were 
surrounded  with  or  intermingled  with  light  objects. 


The  Exposure 

Exactly  the  same  consideration  applies  to  light  objects  but 
in  a  reverse  sense.  So  in  making  this  little  allowance  we 
are  only  doing  what  our  eyes  do  automatically,  and  the 
result  is  more  truthful  because  of  it.  These  considerations 
apply  to  dark  tapestry,  dark  pictures,  bronze  statues, 
wooden  objects  darkened  by  age,  as  oak  screens,  doors, 
&c.,  on  the  one  hand,  and  such  things  as  marble  statues, 
buildings  of  light  stone  or  with  very  light  walls  as  some¬ 
times  in  churches,  pictures  that  are  essentially  light,  draw¬ 
ings  or  prints  done  in  black  lines  on  white  paper,  on  the 
other  hand.  An  open  landscape  with  no  dark  objects  or 
shadows,  and  the  open  sea,  being  subjected  to  the  full 
glare  of  light  from  the  sky,  need  a  reduction  to  about  a 
fourth,  while  the  sky  itself  may  be  given  an  exposure  of 
about  one-eighth  of  what  would  be  given  for  an  average 
object,  when  the  value  found  for  the  light  intensity  is 
the  same. 

If  you  will  paint  upon  a  piece  of  cardboard  the  blackest 
coating  that  you  can  of  the  blackest  paint  or  ink,  and  take 
also  a  piece  of  the  whitest  card  or  paper  that  you  can  get, 
you  will  then  have  represented  the  blackest  black  and  the 
whitest  white  available  for  the  usual  means  of  pictorial 
representation,  whether  photographic  or  otherwise.  In 
the  dark  neither  of  the  cards  will  be  visible  because  then 
everything  will  be  black,  and  black  and  white  are  only 
distinguishable  by  reason  of  the  light  that  shines  upon 
them.  Put  a  lighted  candle  at  a  distance  of  say  two  feet 
from  the  two  cards  and  the  difference  between  the  black 
and  the  white  is  clearly  shown.  But  if  you  could  arrange 
so  that  forty  or  fifty  candles  were  to  shine  upon  the  black 
card  at  exactly  the  same  distance  that  the  single  candle 
illuminates  the  white  card,  the  black  card  would  appear 
whiter  than  the  white  card,  and  if  the  number  of  candles 
was  reduced  to  about  thirty,  then  the  two  cards  would 
appear  of  equal  blackness  or  whiteness,  whichever  you  may 
prefer  to  call  it.  An  absolutely  white  card  would  reflect 

146 


The  Exposure 

all  the  light  that  falls  upon  it  and  an  absolutely  black  card 
would  reflect  none  of  it,  but  absolute  blackness  and  white¬ 
ness  are  impossible,  for  the  whitest  card  does  not  reflect 
all  the  light  and  the  blackest  reflects  some.  We  may 
truthfully  say  that  the  only  difference  between  practical 
blacks  and  whites  is  that  the  blacks  reflect  less  of  the  light 
that  falls  upon  them  than  the  whites.  As  it  is  found  that 
when  about  thirty  times  as  much  light  illuminates  the 
black  card  as  illuminates  the  white  card  they  are  equally 
bright,  it  follows  that  the  only  difference  between  the 
black  and  the  white  is  that  the  former  reflects  about  one- 
thirtieth  as  much  light  as  the  other  when  they  are  equally 
illuminated,  as  they  would  be  under  ordinary  conditions. 
This  range  of  one  to  thirty  is  thus  about  the  maximum 
available  ;  but  if  the  paper  used  is  not  of  the  whitest  and 
the  blacks  are  not  of  the  deepest,  the  difference  possible  is 
at  once  reduced.  And  if  the  paper  is  tinted  or  toned,  the 
possible  range  of  depth  may  be  as  little  as  about  one  to  ten 
or  even  less. 

With  such  a  restricted  power  at  command  it  is  im¬ 
possible  to  represent  without  compromise  the  great  range 
of  brightness  that  is  met  with  in  nature.  If  the  fine  model¬ 
ling  of  the  bright  clouds  is  well  shown,  so  much  of  the 
possible  range  of  brightness  in  the  picture  is  taken  up  by 
it  that  there  is  an  unduly  small  remainder  for  the  land¬ 
scape  and  this  therefore  appears  oppressively  dark.  If 
the  dark  parts  of  the  subject  are  made  the  best  of,  then 
the  brighter  parts  suffer.  This  therefore  is  a  full  justifica¬ 
tion  for  the  adjustment  of  the  exposure  to  suit  the  subject, 
when  that  is  possible,  within  the  comparatively  narrow 
limits  suggested  above. 

This  restriction  of  the  photographer  with  regard  to  the 
extremes  of  black  and  white  at  his  disposal,  leads  some, 
whose  only  aim  is  to  make  pleasing  pictures,  to  invariably 
select  subjects  that  present  very  little  contrast,  and  that 
can  therefore  be  satisfactorily  rendered  by  the  range  of 

147 


The  Exposure 

brightness  available  for  the  print.  They  prefer  grey  days, 
evening  scenes,  photographing  sometimes  even  after  sunset 
when  all  deep  shadows  and  high  lights  have  given  place  to 
a  general  dim  illumination.  From  a  merely  technical 
point  of  view  such  subjects  are  more  easy  to  represent 
satisfactorily  than  those  that  show  more  brilliant  contrasts. 
It  is  easier  to  get  the  values,  or  different  degrees  of  bright¬ 
ness,  correctly  represented  because  there  is  so  small  a 
range  of  them.  Although  it  is  waste  of  time  to  attempt 
the  impossible,  it  is  not  desirable  to  confine  our  endeavours 
to  what  is  easy,  merely  because  it  is  easy. 


1 48 


CHAPTER  IX 

THE  DEVELOPMENT  OF  THE  PLATE 

The  use  of  the  negative  is  to  furnish  a  print  by  putting  a 
piece  of  sensitive  paper  beneath  it  and  then  exposing  it  to 
light.  The  negative  shields  the  paper  where  the  light  is 
not  required  to  act,  and  it  must  therefore  have  a  sub¬ 
stantial  degree  of  opacity.  The  simple  exposure  of  the 
plate  in  the  camera,  as  described  in  the  last  chapter,  pro¬ 
duces  no  visible  effect  upon  it,  and  if  after  the  exposure 
the  sensitive  silver  bromide  were  to  be  dissolved  away, 
there  would  be  nothing  to  be  seen  on  the  glass  except  the 
transparent  film  of  gelatine.  Exposure  alone  under  these 
circumstances  cannot  give  a  negative,  all  that  it  does  is  to 
render  the  silver  salt  less  stable  and  so  give  the  possibility 
of  making  a  negative  by  taking  advantage  of  this  instability. 
It  would  be  quite  possible  under  different  conditions  to 
cause  the  light  to  do  the  whole  of  the  work,  and  this  was 
the  only  method  known  until,  nearly  eighty  years  ago,  the 
production  of  an  invisible,  or  “  latent  "  or  “  developable  " 
image  was  discovered.  When  the  light  acting  on  the  plate 
is  only  required  to  initiate  or  render  possible  the  required 
change,  the  time  necessary  for  the  exposure  in  the  camera 
is  enormously  reduced. 

The  word  development  has  a  general  meaning  as  well 
as  its  technical  meaning  when  used  in  connection  with 
photography ;  and  even  when  it  is  limited  to  our  subject 
it  has  various  meanings  which  must  not  be  confused. 
Niepce  developed  his  bitumen  photographs  a  hundred 
years  ago.  A  film  of  bitumen  similar  to  a  coating  of 

149 


The  Development  of  the  Plate 

varnish  covered  the  surface  of  metal  or  other  material,  and 
was  exposed  to  light  in  a  camera,  until  by  the  action  of  the 
light  those  parts  of  the  film  that  it  acted  on  were  rendered 
less  easily  soluble  in  an  oily  liquid.  This  image  was 
practically  invisible  and  was  '‘developed"  by  applying  the 
solvent  carefully  so  that  it  should  remove  the  more  soluble 
parts  of  the  film  without  interfering  with  the  exposed  and 
less  soluble  parts.  Here  the  whole  of  the  change  was 
effected  by  the  light,  and  the  development  was  simply  the 
removal  of  the  unchanged  parts.  We  shall  find  other 
examples  of  this  kind  of  development  in  the  consideration 
of  printing  processes. 

Daguerreotypes  w^ere  developed,  but  on  quite  a  different 
principle.  The  effect  of  the  light  here  also  was  invisible, 
and  the  image  was  developed  or  made  visible  by  subject¬ 
ing  it  to  the  action  of  mercury  vapour.  The  metal  con¬ 
densed  in  minute  drops  more  readily  on  those  parts  of  the 
plate  where  the  light  had  acted  than  on  the  other  parts. 
Wet  collodion  plates  were  developed  in  a  similar  way,  only 
instead  of  the  deposition  of  metallic  mercury  from  its 
vapour,  metallic  silver  was  deposited  from  its  solution. 
This  kind  of  development  can  be  very  easily  illustrated  by 
breathing  upon  any  ordinary  piece  of  glass.  If  the  glass  is 
quite  clean  an  even  deposit  of  drops  of  moistuie  fiom  the 
breath  will  be  formed,  but  if  the  glass  is  not  clean,  the 
deposit  will  be  irregular  and  uneven.  The  irregularities 
will  often  be  more  conspicuous  as  the  deposited  moisture 
evaporates,  the  moisture  remaining  the  longest  where  it 
was  most  copiously  deposited.  It  is  very  usual  to  test 
for  invisible  impurities  on  the  surface  of  glass  by  breathing 
on  it,  and  in  this  operation  these  invisible  deposits  are 
developed  into  visibility  by  the  deposition  of  the  little  drops 
of  water  upon  them  at  a  different  rate  or  in  differently 
sized  drops  from  its  deposition  upon  the  clean  surface. 
There  is  nothing  essentially  photographic  in  development 
by  dissolving  awray  or  in  development  by  deposition,  they 

150 


The  Development  of  the  Plate 

are  quite  common  operations  used  by  multitudes  of  persons 
who  never  give  a  careful  thought  to  the  matter.  They 
have  doubtless  been  in  use  for  thousands  of  years. 

But  the  kind  of  development  that  we  are  about  to 
consider  is  of  an  essentially  different  character.  It  is 
possible  to  apply  certain  theories  as  to  the  course  of  the 
changes  that  take  place,  and  so  to  compare  this  develop¬ 
ment  with  development  by  deposition,  but  this  is  not  a 
suitable  opportunity  to  endeavour  to  apply  theories  that 
may  or  may  not  stand  the  criticism  of  further  investigation. 
The  difference  in  the  nature  of  this  kind  of  development 
has  always  been  recognised,  and  it  is  called  “  chemical 
to  distinguish  it  from  development  by  deposition,  which  is 
called  “  physical.”  Clearly  in  the  latter  case  there  is  no 
chemical  change  involved  in  the  actual  process  of  develop¬ 
ment  in  any  of  the  substances  concerned ;  the  water 
remains  water,  the  mercury  remains  mercury,  and  the 
silver  remains  silver;  they  are  merely  discriminatingly 
deposited. 

In  the  development  of  an  ordinary  negative,  the  silver 
bromide  that  has  been  rendered  unstable  by  the  action 
of  light  is  decomposed,  its  bromine  is  taken  away  from 
it  and  the  metallic  silver  remains.  This  is  exactly  the 
effect  that  the  light  itself  would  have  if  its  action  were 
allowed  to  continue  under  suitable  conditions.  But  it 
is  not  true  to  say  that  the  light  starts  the  change  and  the 
developer  continues  it,  because  in  that  case  the  exposure 
and  the  development  should  be  interchangeable,  more 
or  less  exposure  with  less  or  more  development  should  lead 
to  the  same  result.  We  have  seen  very  clearly  in  the 
previous  chapter  that  the  facts  are  very  far  from  this,  and 
that  increasing  the  exposure  beyond  a  certain  small  limit, 
so  far  from  facilitating  development,  hinders  it  and  may 
render  it  impossible.  The  opening  of  a  door  will  make 
the  entrance  of  an  audience  possible,  but  it  will  nevei  fill 
the  building.  The  opening  of  two  doors  will  further 

15 1 


The  Development  of  the  Plate 

facilitate  entrance,  but  if  the  people  do  not  go  in  the  place 
will  remain  empty.  So  the  exposure  makes  development 
possible,  but  it  is  not  a  stage  in  the  actual  production  of 
the  image.  It  is  the  developer  that  does  this,  and  the 
light  action  simply  determines  where  it  shall  act.  In  the 
terms  of  the  illustration,  it  opens  the  door  to  this  apart¬ 
ment  but  not  to  that,  it  opens  this  door  wide  that  many 
may  enter  and  the  other  to  a  small  extent  so  that  only 
a  few  can  get  in  there. 

The  first  requisite  of  a  developer  is  that  it  shall  be 
able  to  remove  bromine  from  silver  bromide,  and  the 
second  is  that  it  shall  not  be  able  to  do  so  unaided,  but 
only  when  its  action  has  been  facilitated  by  the  previous 
action  of  light  or  its  equivalent.  Whatever  is  the  prime 
reagent  employed,  it  is  desirable  to  be  able  to  adapt  it  to 
varying  circumstances  by  accelerating  its  action  or  retard¬ 
ing  it,  and  that  it  may  be  caused  to  do  its  work  in  a 
steady  and  well-balanced  manner.  The  mixture  used 
for  developing  therefore  consists  generally  of  the  develop¬ 
ing  agent  proper,  an  accelerator,  and  a  retarder.  But 
why,  it  may  be  asked,  must  there  be  two  agents  so  ap¬ 
parently  opposed  to  each  other  as  these  last  ?  Could  not 
the  accelerator  be  reduced  in  its  quantity  and  the  retarder 
dispensed  with  ?  This  may  be  possible  but  it  is  hardly 
ever  desirable.  As  a  rough  illustration,  we  may  regard  the 
developing  agent  itself  as  the  wind  that  carries  the  ship 
along,  the  accelerator  as  the  sails  that  enable  the  wind 
to  act  more  powerfully  on  the  ship,  and  the  retarder  as 
the  ballast.  The  greater  the  weight  of  ballast  the  more 
sail  will  be  required  to  maintain  the  same  speed,  but 
the  reduction  of  both  cannot  be  carried  beyond  a  certain 
point  without  instability  and  uncertainty  as  to  the  progress 
of  the  vessel.  The  fly-wheel  attached  to  a  steam-engine 
retards  its  movements,  but  it  is  desirable  in  order  to  over¬ 
come  the  irregularities  of  the  movements  that  would  be 
produced  by  the  engine  without  it.  In  an  analogous  way 

J52 


The  Development  of  the  Plate 

the  retarder  is  needed  to  steady  the  reaction  while  the 
accelerator  urges  it  on. 

There  is  one  other  constituent  that  is  necessary  to 
form  a  well-balanced  developing  solution.  The  develop¬ 
ing  agent,  as  it  takes  the  bromine  from  the  silver  bromide 
of  course  combines  with  it,  and  the  product  of  this  com¬ 
bination  almost  always  has  a  deep  colour.  This  if  uncon¬ 
trolled  would  stain  the  negative.  The  final  constituent  of 
the  developer  is  added  to  prevent  this  and  keep  the 
negative  clean.  We  will  now  pass  in  review  each  of  these 
four  constituents. 

First  with  regard  to  the  developing  agent  itself.  We 
have  already  referred  to  the  fact  that  Wedgwood  and 
Davy  at  the  beginning  of  the  last  century  found  that 
leather  was  better  than  paper  for  this  purpose  because 
it  rendered  the  silver  salt  deposited  upon  it  more  sensitive. 
The  Rev.  J.  B.  Reade,  working  on  similar  lines  and 
having  no  more  white  leather,  attempted,  as  he  said, 
to  “tan  paper,"  that  is  to  prepare  paper  in  the  same 
way  that  leather  is  prepared  in  order  to  confer  upon 
the  paper  the  accelerating  influence  found  in  the  leather. 
He  applied  to  the  paper  an  infusion  of  gallnuts,  these 
being  one  of  the  common  sources  of  tannin  used  in 
the  preparation  of  leather.  Fox  Talbot  heard  of  Reade’s 
procedure,  and  discovered  that  the  tannin  or  gallic  acid 
might  be  applied  after  the  exposure,  or  a  part  of  it 
might  be  so  applied,  in  order  to  bring  out  or  develop 
the  image  that  was  not  visible  before  its  application. 
Tannin  may  be  regarded  as  a  compound  of  gallic  acid 
and  glucose,  and  the  acid  is  easily  obtained  from  it. 
When  gallic  acid  is  heated  to  a  moderate  temperature 
it  gives  a  sublimate  of  light  feathery  crystals,  which  at 
first  were  thought  to  be  merely  purified  gallic  acid  but 
were  afterwards  found  to  be  a  different  substance,  what 
we  now  call  pyrogallic  acid  or  pyrogallol.  It  was  in 
this  accidental  way  that  pyrogallic  acid  came  to  be  used 

153 


The  Development  of  the  Plate 

as  a  developer,  and  it  is  strange  that  even  to  the  present 
day  it  is  largely  employed,  for  nothing  superior  to  it 
in  every  way  has  yet  been  found.  Later  on  one  or  two 
other  substances  were  found  serviceable  as  developers, 
but  it  was  not  until  about  twenty  years  ago  that  it  was 
sought  to  discover  the  particular  characteristics  of 
developers  from  a  chemical  point  of  view,  in  order  that 
the  chemist  might  have  a  definite  idea  as  to  the  general 
nature  of  other  substances  that  might  take  the  place  of 
the  pyrogallol.  Clearly  there  was  no  obvious  reason 
why  pyrogallic  and  gallic  acids  should  stand  alone  with 
regard  to  this  power  of  development,  and  it  might 
happen  that  other  reagents  would  be  superior  to  them. 
Doubtless  also  there  was  the  commercial  incentive,  that 
a  better  developer  would  be  profitable  to  the  makei 
of  it. 

All  definite  substances  have  what  chemists  call  a 
constitution,  that  is  their  constituent  parts  can  be  so 
represented  that  the  relationship  that  they  bear  to  each 
other  will  indicate  the  possibilities  of  change  of  that 
particular  substance.  There  are  often  found  to  exist 
several  substances  of  exactly  the  same  composition,  the 
same  elements  in  exactly  the  same  proportions,  but  with 
different  and  sometimes  very  widely  different  properties. 
The  difference  in  such  cases  must  be  due  to  the  relation¬ 
ship  that  the  various  parts  of  the  compound  bear  to 
each  other,  and  by  ascertaining  the  ways  in  which  it  is 
possible  to  change  such  compounds,  the  relationship 
of  the  various  parts  to  each  other  may  be  discovered 
and  definitely  expressed.  By  investigations  on  such  lines, 
the  chemical  characteristics  of  developers  as  a  class 
were  worked  out,  and  as  a  result  a  great  many  efficient 
developers  were  placed  at  the  disposal  of  the  photo¬ 
grapher.  Each  of  these  has  its  own  characteristics,  but 
their  differences  are  not  remarkably  great,  and  for 
practical  purposes  it  is  hardly  worth  while  to  do  more 

154 


The  Development  of  the  Plate 

than  to  divide  them  into  two  classes,  namely,  the  “slow” 
and  the  “  rapid  ”  developers.  The  words  slow  and  rapid 
do  not  refer  to  the  whole  process  of  development,  but 
only  to  the  first  visible  effect  of  the  developer.  When 
a  properly  compounded  slow  developer  is  put  upon  a 
plate,  the  high  lights  of  the  image  appear  first,  then  the 
middle  tones,  and  finally  the  detail  in  the  shadows  ;  but 
with  a  rapid  developer,  the  whole  image  appears  to 
begin  to  come  at  the  same  time  and  very  much  more 
quickly  after  the  application  of  the  solution  than  in  the 
other  case.  But  this  distinction  is  not  always  very 
conspicuous,  and  the  most  rapid  of  the  “  rapid 
developers  may  be  made  to  act  like  a  slow  developer 
by  suitable  means. 

The  accelerator  is  an  alkali,  because  the  developing 
agents  are  more  ready  to  take  up  bromine  or  other 
similar  substances  in  the  presence  of  an  alkali.  Ihose 
who  do  any  practical  photographic  work  might  try  the 
experiment  of  shaking  up  in  a  tube  a  little  pyrogallic 
acid  in  water.  There  will  be  but  little  change.  If  now 
any  alkali,  such  as  some  washing  soda  or  ammonia, 
is  added  and  it  is  shaken  again,  the  solution  will  rapidly 
darken,  and  this  production  of  colour  shows  that  oxygen 
from  the  air  has  been  absorbed  and  combined  with  by 
the  pyrogallic  acid.  In  an  analogous  way,  if  a  pyrogallic 
acid  developer  without  any  alkali  is  put  upon  an  exposed 
plate  it  will  act  very  slowly  indeed,  requiring  perhaps  a 
whole  day  or  more  to  effect  development,  and  in  order  to 
make  it  produce  a  respectable  image  it  will  be  desirable 
to  give  the  plate  a  longer  exposure  in  the  first  instance. 

An  accelerator  may  not  always  be  an  obvious  alkali. 
Acetone  or  formic  aldehyde,  for  example,  are  not  alkalies, 
but  when  added  to  a  developer  they  seem  to  act  as 
accelerators.  The  fact  is  that  they  act  only  indirectly. 
Compounds  of  the  type  to  which  these  belong  will 
combine  with  acid  sodium  sulphite.  Now  the  sodium 

155 


The  Development  of  the  Plate 

sulphite  that  is  put  in  the  developer  may  be  regarded  as 
a  compound  of  acid  sodium  sulphite  and  caustic  soda  ; 
it  can  be  divided  into  these  two  substances,  but  so  long 
as  they  are  together  they  neutralise  each  other.  When, 
however,  acetone  is  added  which  can  combine  with  the 
acid  sodium  sulphite,  the  caustic  soda  becomes  available 
for  the  purposes  of  the  developer.  Trisodium  phosphate 
is  sometimes  used  as  an  accelerator.  This  may  be 
regarded  as  a  compound  of  disodium  phosphate  and 
caustic  soda,  and  when  dissolved  in  water  the  caustic 
soda  is  at  once  available.  Therefore  the  use  of  these 
substances  is  only  an  indirect  way  of  adding  an  alkali,  and 
though  there  may  sometimes  be  an  advantage  in  employ¬ 
ing  them,  it  is  generally  more  satisfactory  to  add  a  simple 
alkali  directly.  Moreover  acetone  and  formic  aldehyde 
are  volatile  and  pungent  substances  that  are  disagreeable 
to  have  to  deal  with,  and  if  inhaled  in  quantity  are  likely 
to  do  injury  to  the  individual.  There  are  a  few  developers 
that  will  work  fairly  satisfactorily  without  any  alkali  to 
accelerate  them,  but  these  generally  contain  in  themselves 
rather  more  of  an  alkaline  character  than  the  others. 

The  retarder  is  almost  always  potassium  bromide.  A 
few  other  substances  have  been  suggested,  but  they  do 
not  meet  with  general  approval.  We  have  likened  the 
retarder  to  the  ballast  of  a  sailing-vessel.  There  are  some 
vessels  that  need  no  ballast,  and  there  are  some  plates 
that  need  no  retarder.  This  is  so  especially  with  slow 
plates,  the  gelatine  of  the  film  is  a  sufficient  retarder. 
But  there  are  other  cases  where  the  photographer  may 
be  deceived.  Just  as  in  building  a  vessel  the  keel  may 
sometimes  be  made  heavy  enough  to  steady  it  without  the 
addition  of  ballast— the  ballast,  so  to  speak,  being  part  and 
parcel  of  the  vessel — so  the  plate  maker  can  put  a  little 
potassium  bromide  into  his  emulsion,  and  the  desirable 
retarder  then  becomes  a  part  of  the  plate,  and  obviously 
need  not  be  added  to  the  developer. 

I56 


The  Development  of  the  Plate 

The  final  constituent  of  the  developer  is  the  stain 
preventer,  which  is  generally  sodium  sulphite.  If  the 
experiment  suggested  above  of  shaking  up  a  solution  of 
pyrogallic  acid  with  an  alkali  be  repeated,  but  with  the 
addition  of  sodium  sulphite,  the  effect  of  the  sulphite  in 
preventing  the  production  of  the  colour  will  be  demon¬ 
strated,  although  the  absorption  of  oxygen  from  the  air 
will  take  place  pretty  much  as  when  without  the  sulphite 
the  solution  became  very  much  darkened.  The  use  of 
sodium  sulphite  for  this  purpose  was  suggested  very  soon 
after  gelatine  plates  began  to  be  generally  employed,  and 
until  then  the  negatives  made  were  badly  coloured  with 
the  staining  matter  produced,  for  it  is  not  simply  the 
bromine  from  the  silver  bromide  but  also  the  oxygen  of 
the  air  that  comes  into  contact  with  the  surface  of  the 
developer,  that  is  taken  up  by  it  and  converts  it,  in  the 
absence  of  sulphite,  into  darkly  coloured  substances. 

After  so  much  preliminary  consideration  of  the  various 
constituents  of  the  developer,  we  must  endeavour  to  follow 
the  course  of  its  action  upon  a  properly  exposed  plate. 
We  have  already  indicated  the  differences  that  will  be 
caused  by  too  little  or  too  much  exposure.  If  the 
developer  is  pyrogallic  acid  and  a  sufficiency  of  sodium 
sulphite  has  been  added  to  prevent  the  production  of 
coloured  substances,  and  if  the  accelerator  has  been  rather 
sparingly  added  so  that  the  total  time  of  development  is 
about  fifteen  minutes,  the  plate  when  placed  in  the 
developer  will  remain  for  two  and  a  half  to  three  minutes 
with  its  creamy  white  surface  unaffected.  Then  a  slight 
darkening  will  be  apparent  at  those  parts  where  the  most 
brilliant  parts  of  the  image  fell,  and  the  darkening  will 
very  slowly  increase.  As  it  does  so  those  parts  where  the 
image  was  rather  less  bright  will  begin  to  appear,  and  as 
these  are  all  gradually  growing  in  density,  the  detail  in 
the  darkest  parts  of  the  image  will  come  out.  Now  the 
various  parts  of  the  image  that  first  appeared  will  be 

*57 


The  Development  of  the  Plate 

invisible  on  the  surface  of  the  plate,  “buried”  as  it  is  said 
because  the  action  on  the  surface  will  be  equal  over  a 
those  parts  of  the  plate,  but  by  holding  it  up  to  the  light 
and  looking  through  it,  it  will  be  seen  that  the  gradation 
of  the  whole  image  has  been  duly  preserved.  It  only 
remains  now  to  dissolve  away  the  silver  bromide  that  has 
not  been  utilised  to  form  the  image  by  allowing  the  plate 
to  remain  in  a  solution  of  sodium  hyposulphite  until  the 
white  creamy  appearance  of  the  film  has  gone  and  e 
black  image  stands  out  clearly  in  a  transparent  and 
colourless  medium.  After  a  thorough  washing  to  remove 
all  the  soluble  matter  from  the  film,  the  plate  is  put  o 


^If  the  subject  photographed  was  of  such  a  character 
that  the  image  had  not  too  great  a  difference  in  brightness 
between  its  brightest  and  its  darkest  parts,  or  if  the  plate 
used  was  one  that  would  not  give  too  much  contrast  with 
such  a  subject,  then  the  development  presents  no  diffi¬ 
culties,  because  it  is  only  necessary  to  let  it  go  on  until 
all  the  bromide  of  silver  that  was  made  developable  by 
the  exposure  has  had  its  bromine  removed  from  it.  If 
the  developer  were  well  balanced  and  the  plate  o  goo 
quality,  the  plate  might  remain  in  the  developer  for  twice 
as  long  as  necessary  without  harm,  for  when  the  develop¬ 
ment  was  complete  there  would  be  no  further  change 
possible  within  a  reasonable  time.  But  if  it  were  possib  e 
io  produce  too  great  a  contrast  in  the  negative  by  reason 
of  the  character  of  either  the  subject  or  the  plate,  then  a 
certain  measure  of  skill  is  necessary  to  know  when  the 
development  has  been  carried  far  enough.  If  the  photo¬ 
grapher  is  dealing  with  subjects  of  very  various  char¬ 
acters  and  wishes  to  render  each  of  them  as  perfectly 
as  possible,  then  whatever  rules  he  may  find  useful  he 
must  depend  finally  upon  his  discretion,  because  the 
density  of  a  negative  cannot  be  measured  as  it  is  being 

developed. 

158 


The  Development  of  the  Plate 

There  are  rules  concerning  development  that  are  some¬ 
times  a  guide  as  to  when  the  operation  has  been  carried 
on  long  enough,  that  is  as  to  when  a  sufficient  degree  of 
contrast  has  been  obtained.  Mr.  Alfred  Watkins  has 
found  that  the  time  required  for  the  first  effect  to  be 
visible  bears  a  definite  proportion  to  the  total  time  of 
development  under  certain  circumstances.  By  noticing 
how  long  the  developer  takes  to  produce  the  first  visible 
result,  this  period  can  be  multiplied  by  the  known  factor 
and  development  carried  on  as  long  as  indicated. 
With  ordinary  developers  this  multiplying  factor  varies 
from  about  four  to  forty,  not  that  the  total  time  of  de¬ 
velopment  varies  to  anything  like  this  extent,  but  that  some 
developers  produce  a  visible  effect  so  very  soon  after 
they  are  poured  on  the  plate,  while  others  may  be  more 
than  ten  times  as  long  in  this  first  stage.  Another  method 
consists  in  finding  how  long  a  specific  developing  solution 
requires  to  develop  a  given  plate  exposed  on  an  average 
subject  and  how  much  this  time  is  affected  by  a  change 
of  temperature,  for  the  higher  the  temperature  the  quicker 
the  action,  as  is  the  rule  in  cases  of  chemical  change.  It 
is  obvious  that  there  is  no  law  involved  in  either  method, 
they  are  only  rules  that  apply  in  average  cases.  If,  how¬ 
ever,  either  of  these  methods  is  implicitly  followed,  the 
probability  is  that  the  average  photographer  will  secure  a 
greater  proportion  of  good  negatives  than  if  he  trusted 
solely  to  his  discretion.  Those  who  have  had  compara¬ 
tively  little  experience  are  so  liable  to  be  misled  by  their 
own  ideas,  that  slavishly  following  an  average  rule  is  less 
likely  to  lead  to  misfortune.  There  is  also  another  con¬ 
sideration,  namely,  that  the  greater  number  of  photo¬ 
graphers  work  in  grooves,  their  work  is  all  of  very  much 
the  same  sort.  The  professional  portraitist  has  his  style, 
the  amateur  generally  settles  down  to  more  or  less  of 
one  class  of  subject,  and  often,  too,  only  photographs  in 
very  nearly  the  same  sort  of  weather.  Then  the  uniform 

159 


The  Development  of  the  Plate 

method  of  development  is  still  more  likely  to  yield  the 

beStTheetotal  ttmerequired  for  development  can  be  varied 
within  very  wide  limits,  from  two  or  three  minutes  up  to 
Tom:  hours,  for  example,  according  as  the  developer  .s 
diluted,  restrained,  or  contains  little  accelator.  Whe 
many  negatives  have  to  be  developed,  it  is  possible  that 
"he  total  time  required  may  be  shortened  by  Prolonging 
the  time  necessary  for  each  plate,  because  by  so  doing  the 
manipulation  may  be  facilitated.  This  fact  has  led  to  the 
introduction  of  various  tanks  and  similar  devices,  in  which 
a  considerable  number  of  plates  or  films  are  Pla“d 
racks  or  suitable  supports  and  allowed  o  remain  there  in 
a  diluted  developer  for  the  requisite  time.  Th K°da.b 
Company,  in  1903,  issued  a  developing  mac  >n 
roll  films  in  which  the  whole  strip  of  flexible  film, ,  Me 
exposure,  was  wound  off  its  roller  on  to  a  spindle  so  that 
it  lay  against  an  apron  with  ridges  at  its  sides  to  keep  1  s 
convolutions  apart,  and  was  then  slowly  turned  round  u 
the  dilute  developer  for  the  requisite  time.  Instead  of 
this  continuous  rotation,  the  coiled  film  is  now  put  into 
a  cylindrical  vessel  with  a  tight  lid,  and  this  is  turne  over 
two  or  three  times  during  the  necessary  period  lh  is 

tank  method  of  development  is  often  so  arranged  that 
the  greater  part  if  not  all  of  the  operation  may  be 
carried  out  in  daylight  without  risk  of  accidental  exposur  | 

of  the  plates  or  films.  .  . 

Whatever  the  method  of  work,  the  aim  of  the  p 
grapher  should  be  to  get  the  image  of  his  negative  *° 
consist  of  pure  silver,  and  the  gelatine  m  which .  he 
particles  lie  should  be  clean.  To  secure  this  the  fixing 
must  be  complete,  that  is  the  sodium  hyposu  phite  must 
be  allowed  ample  time  to  dissolve  out  the  bromide  of 
silver  that  has  not  been  utilised  in  the  production  of  the 
image,  and  the  subsequent  washing  must  be  prolonged 
that6  all  the  chemical  substances  in  the  gelatine  from  the 

160 


Maeterlinck 

An  example  of  Modern  Poitraiture, 


E.  0.  Hoppe. 


The  Development  of  the  Plate 

developer  and  the  fixing  bath  may  be  thoroughly  removed. 
It  is  easy  to  talk  of  thoroughly  washing  a  gelatine  plate. 
By  soaking  it  in  fresh  portions  of  water  repeatedly,  a 
proportion  of  the  soluble  matter  is  removed  each  time,  but 
never  more  than  a  portion,  so  that  an  absolute  clearing 
out  of  the  soluble  matter  is  not  possible.  But  such  an 
action  is  rarely  if  ever  so  simple  as  it  might  be  thought 
to  be.  The  gelatine  and  the  silver  image  itself  tend  to 
retain  small  quantities  of  materials  with  which  they  are 
in  contact,  perhaps  in  something  like  the  same  way  that 
a  fabric  retains  the  dye  with  which  it  is  coloured.  It  has 
been  stated  that  the  silver  image  holds  to  itself  a  little  of 
the  bromide  of  silver  that  one  wishes  to  remove  in  fixing 
the  plate,  and  that  this  bromide  of  silver  cannot  be  got 
away.  This  statement  needs  confirmation  ;  but  if  true,  the 
bromide  of  silver  retained  must  be  a  very  small  amount 
when  the  negative  is  properly  made. 

But  there  is  another  interfering  substance  that  is  often 
present  because  of  insufficient  care  on  the  part  of  the 
photographer.  We  have  seen  that  the  developer  when 
acting  takes  bromine  from  the  exposed  bromide  of  silver, 
and  that  the  bromine  when  it  combines  with  the  developer 
converts  it  into  coloured  substances,  to  avoid  which  the 
sodium  sulphite  is  added.  Some  people  seem  to  be  afraid 
of  sodium  sulphite,  and  in  many  formulae  that  are  re¬ 
commended  it  is  put  down  in  insufficient  quantity  to  do 
the  work  that  is  required  of  it.  In  such  cases  colouring 
matter  is  produced,  and  as  the  bromine  leaves  the  silver 
and  goes  to  the  developer  and  forms  this  colouring  matter, 
it  is  produced  exactly  where  the  silver  image  is  produced 
and  is  roughly  proportional  to  it.  There  is  thus  a  com¬ 
pound  image  of  metallic  silver  and  colouring  matter  from 
the  developer,  and  that  this  is  so  can  be  easily  proved  by 
dissolving  away  the  metallic  silver,  when  the  image  in  this 
colouring  matter  remains  and  is  clearly  visible.  This 
colouring  matter  is  undesirable  for  many  reasons  which 

161  l 


The  Development  of  the  Plate 

cannot  be  fully  explained  here.  The  negative  in  which 
it  exists  cannot  be  depended  on  in  any  way.  It  may 
sometimes  be  advantageous  for  non-critical  work,  when 
because  of  the  plate  being  poorly  coated,  or  the  subject 
very  wanting  in  contrast,  or  the  treatment  of  it  defective, 
there  is  too  little  silver  to  furnish  a  suitably  dense  image. 
But  if  the  silver  image  is  too  thin  and  it  is  clean,  there 
are  methods  of  working  with  it  or  increasing  its  density 
that  are  not  subject  to  any  of  the  disadvantages  of  the 
presence  of  the  stain  image.  To  avoid  the  production  of 
staining  matter,  it  is  desirable  to  use  sufficient  sulphite  in 
the  developing  and  to  add  a  little  sulphite  and  alkali  o 
the  fixing  solution  of  sodium  hyposulphite.  If  then  a  little 
staining  matter  has  been  formed,  the  alkaline  solution 
tends  to  keep  it  in  a  soluble  condition  and  to  permit  o 
its  removal  by  washing. 

When  the  developing  solution  darkens  because  o 
undue  exposure,  for  the  air  will  act  on  it  in  a  similar 
manner  to  the  bromine  that  it  takes  from  the  silver 
bromide,  or  because  of  insufficient  sulphite,  the  gelatine 
also  is  generally  stained  of  a  more  or  less  uniform  tint 
Such  a  stain  retards  the  passage  of  light  through  it,  and 
hence  is  detrimental  in  the  printing  process.  The  old 
empirical  method  of  attempting  to  cure  such  a  plate  was 
by  the  application  to  it  of  an  acid  liquid,  and  many  such 
« clearing  baths"  were  recommended  in  the  early  days 
of  gelatine  plates,  and  a  few  have  survived  even  to  the 
present.  In  general,  the  effect  of  the  acid  is  not  to  remove 
the  staining  material,  but  to  change  its  colour  to  a  lighter 
tint  which  of  course  is  less  obvious— brown  to  yellow,  for 
example.  At  the  same  time,  it  tends  to  make  the  colouring 
matter  insoluble,  and  so  may  do  more  harm  than  good. 
The  proper  method  of  treating  a  developer  stain  is  to 
wash  it  away  by  the  repeated  application  of  water  to  which 


The  Development  of  the  Plate 

a  little  alkali  (preferably  caustic  soda)  has  been  added. 
The  alkali  keeps  it  soluble  and  visible,  and  when  all  that 
can  be  removed  in  this  way  has  been  washed  away,  the 
small  residue  of  stain  will  probably  be  so  slight  as  to  be 
invisible,  and  if  in  greater  quantity  will  not  be  lightened 
in  colour  by  any  so-called  “  clearing  solution.” 


CHAPTER  X 

FINISHING  THE  NEGATIVE 


It  is  an  entirely  mistaken  though  very  general  idea  that 
when  a  plate  has  been  exposed,  developed,  fixed,  washed, 
and  dried,  the  negative  ought  to  be  perfect,  and  that  if  it  is 
not,  it  is  either  the  fault  or  the  misfortune  of  the  photo¬ 
grapher.  It  is  also  a  mistaken  and  common  idea  that  it 
is  possible  to  define  a  perfect  negative  in  definite  terms 
which  express  certain  relationships  between  it  and  the 
object  photographed  or  between  the  print  that  it  will  yield 
and  the  object,  and  that  therefore  there  is  only  one  kind  of 
negative  that  is  perfect  for  any  given  object  under  definite 
conditions. 

A  perfect  negative  is  neither  more  nor  less  than  one  that 
will  do  exactly  what  the  photographer  wants  it  to  do,  and 
the  skill  of  the  photographer  is  shown  in  his  ability  to 
produce  exactly  what  he  desires.  Probably  there  is  not 
one  negative  in  a  thousand  or  in  many  thousands  of  those 
that  are  made  that  is  other  than  just  what  happens  to 
come  as  the  result  of  a  prescribed  routine.  A  person 
whose  work  is  of  this  kind  only,  is  no  more  entitled  to  be 
called  a  photographer  than  one  is  entitled  to  be  called 
a  geometrician  because  he  can  neatly  draw  geometrical 
figures.  A  skilful  worker  must  at  least  have  control  over 
his  tools  ;  but  this  alone  is  not  sufficient,  for  a  machine 
might  have  a  far  more  perfect  control  than  he  by  a  life  of 
practice  could  ever  hope  to  attain.  He  must  know  in 
what  direction  to  exercise  the  control,  and  it  is  in  this  that 
he  becomes  superior  to  the  machine  and  rises  from  the 


Finishing  the  Negative 

position  of  a  mechanic  to  that  of  an  artisan  or  artist.  A 
large  proportion  of  the  photographs  that  are  produced  are 
merely  machine  made,  and  poor  at  that,  because  the 
machine  is  a  human  machine  with  all  its  wonderful 
adaptability  uncontrolled.  A  photographer,  to  be  worthy 
of  the  name,  must  be  able  to  form  a  definite  idea  as  to 
what  he  wants,  and  then  be  able  to  work  straight  towards 
the  realisation  of  his  idea.  To  this  there  is  no  exception, 
for  it  applies  equally  to  the  getting  of  a  record  of  the  move¬ 
ments  of  an  instrument,  to  the  photography  of  a  spectrum, 
and  to  the  production  of  photographs  which  are  generally 
distinguished  as  a  pictorial."  By  control  in  this  connec¬ 
tion  we  mean  the  power  to  apply  and  utilise  for  definite 
ends  photographic  instruments  and  methods,  and  not  the 
ability  to  mix  with  the  photography  the  methods  of  other 
arts.  It  is  necessary  to  add  this  explanation,  because  the 
word  “  control "  is  so  often  used  in  connection  with 
photography  in  this  latter  sense,  and  too  often  the 
“  control "  does  badly  what  photography  would  have 
done  well.  Such  productions,  of  course,  are  not  photo¬ 
graphs. 

We  sometimes  hear  it  said  of  a  negative  that  it  is  fine 
or  perfect,  but  that  it  will  not  give  a  good  print.  One 
might  just  as  well  say  of  a  watch  that  it  is  perfect,  but  that 
it  will  not  keep  good  time.  But  look  at  it,  they  will  say, 
there  is  not  a  spot  upon  it,  it  is  brilliant,  full  of  detail, 
clean,  and  altogether  technically  perfect.  If  now  you  will 
ask  the  same  individual  to  buy  a  watch  because  the  case  is 
brilliant,  full  of  wheels,  with  not  even  a  speck  of  dust  to  be 
seen,  and  “technically  perfect,"  he  will  think  that  either 
|  you  are  a  fool  or  consider  him  to  be  one.  Now  a  watch 
:  has  only  one  duty  to  perform,  namely,  to  keep  good  time 
over  a  sufficiently  long  period,  and  time  is  the  same  to  all 
persons.  But  a  negative  may  be  good  to  one  person 
,  because  it  serves  his  purpose,  and  useless  to  another 
because  his  aim  is  different.  The  skilful  photographer 


Finishing  the  Negative 

should  be  able  to  produce  any  kind  of  negative  from  any 
subject  within  the  range  of  the  possibilities  that  photo¬ 
graphy  allows.  For  this  purpose  the  negative  made  as 
already  described  has  often  to  be  subjected  to  other 

treatment.  . 

The  negative  consists  of  particles  of  silver  distributed 
through  the  gelatine  film  in  such  a  manner  that  they 
represent  the  form  of  the  object  as  depicted  by  the  lens, 
and  by  their  density,  the  variations  in  the  brightness  or 
whiteness  of  its  different  parts.  These  particles  cannot  be 
moved  from  one  place  to  another  in  the  film  ;  therefore  at 
this  stage  there  remains  no  power  to  alter  the  u  drawing 
of  the  picture;  that  is  settled  once  for  all  when  the  ex¬ 
posure  is  made.  It  is  only  in  the  density  of  the  various 
parts  that  change  is  possible,  and  this  is  effected  by  adding 
to  the  silver  to  increase  its  density,  or  by  dissolving  some 
of  the  silver  away  in  order  to  reduce  it.  The  first  process 
is  called  “intensification”  and  the  second  “reduction. 

If  a  negative  is  so  thin  in  its  densest  parts  that  the  light  is 
very  little  diminished  by  it,  and  consequently  the  print 
shows  very  little  difference  between  the  darkness  of  one 
part  and  another— the  print,  that  is,  is-flat  and  dull— then  it 
is  desirable  to  increase  the  density  of  the  negative.  On 
the  other  hand,  the  negative  may  have  such  excessive 
contrast  that  the  more  transparent  parts  will  give  a  good 
result  on  the  print  before  the  light  has  sufficiently  pene¬ 
trated  the  darker  parts,  so  that  a  print  at  this  stage  of 
exposure  would  show  patches  of  white  without  detail  or 
gradation  ;  and  if  the  exposure  is  continued  until  the 
darker  parts  of  the  negative  give  a  suitable  result  on  the 
print,  the  effect  under  the  thinner  parts  will  be  buried  in 
blackness.  In  this  case  it  may  be  desirable  to  “reduce* 
the  negative.  The  need  for  the  reduction  of  the  negative 
is  generally,  indeed  one  might  say  always,  due  to  an  error  of 
either  exposure  or  development,  and  when  intensification 
is  necessary  it  may  be  also  due  to  error  in  the  previous 

1 66 


. 


f 


Finishing  the  Negative 

work,  though  often  it  is  otherwise.  The  subject  may 
present  such  uniformity  of  brightness  that  its  image  will 
not  give  sufficient  contrast,  and  in  experimental  and 
scientific  work  it  may  be  desirable  to  increase  the  maxi¬ 
mum  contrast  that  can  be  obtained  by  development. 

In  order  to  understand  the  effect  of  these  processes, 
it  must  be  borne  in  mind  that  in  a  normally  exposed  and 
developed  negative  the  bulk  of  the  particles  of  silver  that 
represent  the  detail  of  the  thinner  parts  of  it — that  is,  the 
shadows  and  darker  parts  of  the  object — lie  nearer  to  the 
surface  of  the  gelatine  film,  and  that  as  the  deposit 
increases,  the  particles  extend  deeper  down  in  the  film. 
In  the  treatment  of  a  gelatine  plate  every  action  begins 
at  the  outer  surface  of  the  film,  and  therefore  its  effects 
show  there  first,  and  passes  deeper  into  the  film  as  the 
action  is  continued.  If  therefore  by  any  means  the  outer 
surface  of  the  film  is  removed  as  if  it  were  sciaped  off, 
there  will  be  carried  away  a  larger  proportion  of  the 
deposit  of  silver  particles  where  there  is  least  deposit  than 
where  the  deposit  is  thicker.  The  thinner  parts  may  be 
made  so  thin  that  they  are  almost  obliterated,  while  the 
denser  parts  lose  only  a  small  proportion  of  their 
substance  by  such  a  process.  Thus  the  action  is  not 
proportional  in  the  various  parts  of  the  plate  and  the 
character  of  the  gradation  is  altered.  The  difficulty  with 
regard  to  this  alteration  is  that  it  is  uncertain  and  iiregulai, 
and  it  is  impossible  to  tell  from  the  resulting  image  what 
the  original  negative  was,  or  from  the  original  what  it  will 
be  if  subjected  to  such  treatment.  A  method  that  cannot 
be  depended  upon  is  obviously  one  to  be  avoided,  for  in 
all  sound  workmanship  it  is  not  the  possibility  of  success 
with  a  hope  for  a  lucky  result  that  is  sought  after,  but  the 
certainty  of  success. 

There  is  one  case  where  such  a  method  of  reduction 
.is  safe  and  good.  In  the  photography  of  a  subject  that 
is  only  black  and  white — that  is,  free  from  shadows  or  half 

167 


Finishing  the  Negative 

tones,  like  a  pen-and-ink  drawing  on  white  cardboard — 
it  is  desirable  to  get  a  good  deposit  on  development  to 
represent  the  white  board,  and  none  where  the  image  of 
the  black  lines  falls.  But  if  the  blackness  is  not  as  intense 
as  it  might  be,  or  if  the  exposure  has  been  a  little  too  long, 
there  may  result  a  slight  deposit  where  none  was  desired, 
and  this  deposit  may  be  cleared  away  by  reduction. 
Something  will  be  lost  from  the  denser  parts  of  the 
negative,  but  probably  not  very  much,  and  as  the  cardboard 
is  equally  white  all  over  the  negative  is  equally  dense, 
except  where  the  lines  are,  and  there  is  no  gradation  to 
falsify. 

Exactly  the  same  considerations  apply  when,  instead 
of  rubbing  away  the  surface  of  the  gelatine,  a  liquid  is 
applied  to  the  negative  that  will  dissolve  the  silver  out 
from  it.  This  begins  to  act  at  the  surface,  and  therefore 
dissolves  away  a  larger  proportion  of  the  thinner  deposits 
that  lie  chiefly  near  the  surface  than  of  the  denser  deposits 
that  penetrate  more  deeply  into  the  film.  The  unpro¬ 
portional  character  of  the  action  will  be  increased  if  the 
solvent  acts  very  quickly  in  proportion  to  the  time  that 
it  requires  to  penetrate  the  film  ;  and  conversely,  if  it  were 
possible  to  get  a  solvent  that  was  slow  in  action  but  quick 
to  pass  in  and  out  of  the  film,  so  that  every  particle  of 
silver  was  always  surrounded  by  the  solvent  in  the  same 
condition,  then  every  particle  would  be  subjected  to  the 
same  action  and  the  reduction  would  be  proportional 
throughout.  It  is  possible  to  approximate  somewhat  to 
this  condition,  and  some  reducing  reagents  have  been 
stated  to  give  a  proportional  effect,  but  such  a  result  can 
never  be  certainly  obtained. 

There  are  a  great  many  substances  that  may  be  used 
to  make  the  image  on  the  negative  thinner.  The  surface 
of  the  dry  gelatine  may  be  rubbed  away  by  means  of  a 
rag  moistened  with  alcohol,  or  u globe  polish”  may  be 
used,  or  the  surface  of  the  gelatine  may  be  softened  and 

1 68 


Finishing  the  Negative 

made  slimy  by  the  application  of  an  alkali  or  acid,  and 
then  wiped  off.  To  dissolve  the  silver  it  may  be  first 
converted  as  far  as  necessary  into  chloride  of  silver  by 
means  of  a  solution  of  ferric  chloride,  and  the  chloride 
of  silver  may  be  dissolved  in  a  solution  of  sodium  hypo¬ 
sulphite — -the  ordinary  fixing  bath.  The  ferric  chloride 
and  sodium  hyposulphite  cannot  be  mixed  in  order  to 
dissolve  the  silver  immediately  it  is  changed  into  the 
soluble  compound,  because  they  would  act  on  each  other 
and  become  inactive,  but  this  result  may  be  obtained  by 
using  ferric  oxalate  or  potassium  ferricyanide  instead  of 
ferric  chloride.  The  silver  is  then  changed  into  silver 
oxalate  in  the  first  case  and  into  silver  ferrocyanide  or 
ferricyanide  in  the  second  case,  and  these  compounds 
dissolve  immediately  they  are  formed,  being  in  the 
presence  of  the  sodium  hyposulphite.  Ammonium  per¬ 
sulphate  is  often  used  as  a  reducer,  and  it  is  usually 
thought  to  act  either  proportionally  throughout,  or  else 
to  a  greater  extent  on  the  denser  deposits  than  on  the 
thinner  deposits.  But  as  it  is  not  known  how  it  acts, 
and  as  the  nature  of  the  material  that  is  left  to  form  the 
image  has  not  been  determined,  it  is  not  advisable  to 
apply  it  to  any  negative  of  value.  There  are  many  other 
substances  that  can  be  used  to  dissolve  away  some  of  the 
silver  without  injuriously  affecting  the  gelatine. 

It  is  desirable  to  avoid  the  reduction  of  negatives, 
not  because  it  is  an  added  operation,  but  because  the 
result  is  always  uncertain.  It  is  uncertain  because  it 
is  partial  (if  the  action  were  made  complete,  there  would 
be  no  image  left),  and  because  it  is  partial  it  is  unpro¬ 
portional  and  variable.  Circumstances  are  very  different 
in  the  case  of  intensification.  Here  the  action  can  be 
made  complete  because  it  is  possible  to  ensure  that 
every  particle  of  silver  is  acted  on  in  exactly  the  same 
way,  and  being  complete  the  result  can  be  made  strictly 
proportional  and  definite.  Knowing  exactly  what  and 

169 


Finishing  the  Negative 

how  much  of  it  has  been  added  to  each  Par‘lcj®  °l 
silver,  it  is  possible  to  discover,  if  we  should  wish  to  do 
so,  what  the  negative  was  before  the  treatmen  ,  or 
know  exactly  what  any  negative  will  become  by  t 
treatment.  It  must  not  be  thought  that  every  me  hod 
of  intensification  is  of  this  desirable  character.  M  y 
of  those  who  have  worked  at  photographic  pro 
have  had  no  sound  principles  to  guide  them,  and  they 
have  judged  of  the  results  that  they  obtained  merely  by 
the  appearance.  If  a  negative  was  thin  and  the  app  i- 
cation  of  anything  made  the  image  appear  denser  with¬ 
out  destroying  the  gelatine  or  causing  spots  or  stains,  the 
substance  used  was  recommended  as  a  good  intensities 
even  in  the  total  absence  of  any  knowledge  as  to 
how  it  acted  to  produce  the  result  or  of  what  substance 
the  final  image  consisted.  The  inevitable  result  of  this 
was  that  negatives  as  intensified  had  images  consisting 
of  all  sorts  of  things  and  mixtures  of  things,  some 
volatile,  some  soluble,  some  coloured,  and  many  un¬ 
stable.  And  it  naturally  follows  that  many  valuable 
negatives  have  become  useless  from  changes  in  them 
that  might  have  been  foreseen  if  any  one  had  properly 
examined  the  process  before  it  was  applied  to  practical 


For  general  purposes  in  intensification  the  materia 
added  to  the  silver  particles  that  constitute  the  image 
should  be  stable,  black  or  grey— that  is,  not  coloured— the 
process  should  preferably  allow  of  it  being  added  in 
definite  quantities,  and  there  must  not  be  any  tendency 
for  the  solution  used  to  dissolve  any  part  of  the  image. 
In  some  special  cases  the  deposit  may  be  coloured  with¬ 
out  disadvantage,  but  the  very  fact  that  it  is  coloure 
shows  that  it  is  more  opaque  to  light  of  some  co  our 
than  of  others,  and  that  it  will  on  that  account  gi 
different  results  with  various  printing  methods.  A 
coloured  image  is  always  liable  to  be  a  disadvantag 

170 


Finishing  the  Negative 

because  the  light  that  passes  through  it  on  account  of 
its  colour  may  prove  an  annoyance. 

Many  of  the  solutions  that  are  recommended  for 
intensification  will  dissolve  a  part  of  the  substance  or 
substances  that  have  been  added  to  the  original  silver, 
and  sometimes  even  a  part  of  the  silver  itself,  so  that 
there  are  two  opposing  actions  going  on  at  the  same 
time.  While  the  silver  is  being  added  to  in  the  lower 
strata  of  the  film,  this  action  has  been  completed  at  the 
surface,  and  the  product  here  is  being  dissolved  away. 
The  net  result  therefore  is  uncertain,  and  it  is  impossible 
to  get  a  proportional  action. 

The  most  usual  method  of  adding  to  the  silver  of  the 
image  is  to  put  the  plate  into  a  solution  of  a  compound 
from  which  the  silver  will  take  a  constituent  which  will 
form  an  insoluble  compound  with  the  silver,  while  the 
residue  of  the  reagent  is  also  insoluble.  Ferric  chloride 
is  a  compound  of  iron  and  chlorine  from  which  silver 
can  take  some  of  the  chlorine  to  produce  insoluble 
silver  chloride.  But  the  compound  of  iron  and  chlorine 
that  is  left  is  soluble  in  water,  and  therefore  is  not 
deposited  on  the  silver.  If  the  ordinary  chloride  of 
copper  is  used,  silver  chloride  is  produced  as  before, 
and  the  copper  chloride  that  is  left  is  insoluble  in  water 
and  does  remain  where  it  is  produced,  in  close  contact 
with  the  silver.  That  is,  it  remains  to  a  greater  or  less 
extent,  for  it  is  so  easily  changed  by  the  air  to  com¬ 
pounds  that  are  soluble  or  partially  soluble,  that  the 
amount  remaining  is  not  definite.  Mercuric  chloride, 
sometimes  called  corrosive  sublimate,  is  also  able  to 
give  some  of  its  chlorine  to  silver,  and  it  has  the  great 
advantage  that  the  mercurous  chloride  that  is  left  is 
quite  insoluble  and  not  acted  on  by  the  air,  so  that  it 
remains  completely  with  the  silver  chloride,  the  two 
chlorides  combining  to  form  a  definite  compound  which 
will  not  change  at  all,  however  long  the  corrosive  sub- 

171 


1 


Finishing  the  Negative 

limate  solution  is  allowed  to  remain  in  contact  with  it. 
By  looking  at  the  negative  that  has  been  treated  in  this 
way  with  a  magnifying  glass,  it  will  be  clearly  seen  t  lat 
the  original  silver  particles  are  notably  enlarged,  for  they 
have  had  considerably  more  than  twice  their  weight  of 
material  added  to  them.  But  this  double  chloride  of 
silver  and  mercury  is  white,  and  if  the  negative  at  this 
stage  were  used  for  printing,  it  would  be  found  that  in 
spite  of  the  greatly  increased  quantity  of  material  that 
now  forms  the  image,  and  the  consequent  smaller  spaces 
between  the  particles,  more  light  would  pass  through 
it  than  before  because  the  white  particles  reflect  it,  or 
pass  it  on,  so  much  more  freely  than  the  black  particles. 
On  this  account,  for  practical  purposes  the  negative  is 
now  worse  than  at  first.  It  is  necessary  to  change  the 
white  image  into  a  black  one,  and  for  this  purpose  in¬ 
numerable  reagents  have  been  proposed  and  many  ex¬ 
tensively  used  without  any  knowledge  of  the  character 
of  the  change  they  brought  about  or  the  composition  of 
the  black  material  that  they  leave  to  form  the  image. 

There  is  only  one  substance  that  has  been  proved  to 
effect  a  definite  and  simple  and  advantageous  change. 
Ferrous  oxalate  merely  takes  away  the  chlorine  from  the 
white  compound  of  silver,  mercury,  and  chlorine,  and 
leaves  the  whole  of  the  silver  and  the  whole  of  the 
mercury  together  as  a  black  or  greyish  black  deposit.  The 
ferrous  oxalate  can  do  nothing  else  than  effect  this  simple 
change  however  long  it  may  remain  on  the  negative,  and 
there  is  no  circumstance  that  is  likely  to  interfere  with  the 
change  or  the  product.  The  removal  of  the  chloiine 
reduces  the  size  of  the  particles,  but  all  the  original  silver 
remains  with  the  addition  to  it  of  nearly  twice  its  weight 
of  mercury,  and  as  the  mixed  metals  are  of  the  same 
colour  as  the  original  silver,  the  image  is  now  more  able 
to  resist  the  passage  of  light  through  it  than  it  was  at  the 
first.  This  process  of  intensification  has  the  advantage 

172 


Finishing  the  Negative 

that  it  may  be  repeated  as  often  as  desired  on  the  same 
negative,  each  operation  adding  a  definite  proportion  of 
mercury  to  the  image. 

Two  very  favourite  reagents  for  blackening  the  white 
image  produced  by  mercuric  chloride,  are  (i)  ammonia 
and  (2)  potassium  cyanide  dissolved  in  water  with  as  much 
silver  cyanide  dissolved  in  it  as  it  will  take  up.  It  is  not 
necessary  to  trace  the  action  of  each  of  these.  They  both 
have  a  solvent  action  on  the  product  of  the  first  change, 
and  so  begin  to  thin  the  image  as  soon  as  the  blackening 
has  taken  place.  Ammonia  dissolves  away  some  of  both 
the  metals,  and  leaves  the  remainder  in  the  form  of  black 
compounds  of  complex  and  variable  composition.  The 
cyanide  solution  leaves  to  form  the  image  a  mixture  of  silver 
as  metal,  chloride  of  silver,  cyanide  of  silver  and  cyanide  of 
mercury.  Such  mixtures  could  not  be  expected  to  remain 
long  without  change ;  nor  do  they.  The  image  that  ammonia 
leaves  can  generally  be  found  to  have  altered  within  a  week 
or  two  if  the  density  of  the  plate  is  suitably  measured.  These 
methods  are  not  reliable,  although  they  are  so  much  used. 
They  are  of  service  in  technical  work,  in  the  production 
of  negatives  of  simple  black  and  white  subjects  if  the 
negatives  are  not  required  to  last  longer  than  for  a  few 
months. 

There  is  another  method  of  intensifying  negatives  that 
is  exactly  similar  in  principle  to  the  mercury  method,  in 
which  a  ferricyanide  is  used  instead  of  a  chloride.  1  he 
changes  that  take  place  are  a  little  more  complicated, 
because  instead  of  the  simple  substance  chlorine  we  deal 
with  a  complex  substance  that  contains  iron,  carbon,  and 
nitrogen.  Any  metal  that  forms  a  soluble  ferricyanide 
and  an  insoluble  ferrocyanide  is  available  for  this  method, 
but  uranium  and  lead  are  the  two  chiefly  employed.  After 
treatment  the  image  consists  of  a  mixture  of  the  ferro- 
cyanides  of  the  added  metal  and  silver,  a  great  deal  of  new 
material  being  added  to  the  original  silver.  If  uranium 

173 


Finishing  the  Negative 

ferricyanide  is  used,  the  silver  image  is  converted  into  a 
much  denser  reddish  brown  image,  which  has  the  dis¬ 
advantage  of  being  coloured  and  allowing  a  greater  pro¬ 
portion  of  the  red  constituent  of  light  than  of  the  green 
and  blue  to  pass  through  it.  Such  a  partial  action  is 
always  undesirable,  because  various  sensitive  mateiials 
used  for  printing  are  affected  differently  by  the  different 
constituents  of  light,  and  it  is  preferable  to  make  negatives 
that  may  be  used  with  success  for  any  printing  process 
that  may  be  desired.  When  lead  ferricyanide  is  used,  the 
resulting  image  is  white,  as  in  the  case  of  the  mercury 
methods,  and  it  is  very  dense,  for  the  silver  gets  about  three 
times  its  weight  of  lead  added  to  it  in  addition  to  all  the 
iron,  carbon,  and  nitrogen,  that  are  necessary  to  form 
the  ferrocyanides  of  both  metals.  The  white  image  is 
blackened  to  render  it  serviceable  by  changing  the  two 
metals  into  their  sulphides,  which  is  easily  done,  but  how¬ 
ever  treated  the  image  is  so  very  much  denser  than  the 
original,  that  the  method  is  rarely  suitable  for  practical 
purposes. 

It  is  possible  to  increase  the  density  of  the  image  of  a 
negative  that  is  too  thin  by  adding  more  silver  to  it.  In 
the  development  of  a  wet  collodion  plate  the  visible  image 
is  produced  by  the  deposition  of  silver  that  is  in  solution 
in  the  developer  upon  the  invisible  image  that  has  been 
produced  by  light.  The  silver  that  is  in  the  solution  is 
just  upon  the  verge  of  separation  from  it,  and  the  dis¬ 
turbance  caused  by  the  presence  of  the  silver  salt  in  the 
unstable  condition  caused  by  its  exposure  to  light,  de¬ 
termines  the  deposition  of  the  silver  in  its  immediate 
presence.  The  metallic  silver  itself  that  forms  the  visible 
image  is  also  able  to  cause  the  deposition  of  more  silver 
upon  it,  and  if  the  image  has  not  become  dense  enough 
by  the  time  the  stability  of  the  developing  solution  has 
broken  down  and  it  begins  to  deposit  silver  without  the 
presence  of  any  extraneous  cause,  this  solution  can  be 


By  permission  of  the  T/iornton-Pickard  Manufacturing  Co.,  Ltd. 


Finishing  the  Negative 

thrown  away  and  a  new  one  applied.  .  And  so  the  density 
can  be  piled  up  to  almost  any  extent.  The  silver  image 
in  a  gelatine  plate  can  be  added  to  in  a  similar  manner, 
but  this  method  of  increasing  density  is  not  to  be  preferred. 
It  is  more  subject  to  irregularities  in  gelatine  than  in 
collodion,  and  the  photographer  is  dependent  on  the 
appearance  of  the  image  for  an  indication  as  to  when  to 
stop  the  action. 

If  we  were  to  consider  all  the  possible  methods  by 
which  the  silver  image  of  a  gelatine  plate  may  be 
strengthened,  we  should  have  an  almost  endless  task.  But 
for  the  sake  of  showing  what  great  and  varied  resources 
are  at  the  disposal  of  the  photographer  who  understands 
his  subject,  it  may  be  mentioned  that  the  use  of  mercuric 
iodide  gives  rise  to  numerous  methods,  that  platinum  may 
be  deposited  on  the  image,  though  its  deposition  is  accom¬ 
panied  by  other  changes,  and  that  if  a  simple  solution  of 
pyrogallic  acid  is  put  on  to  the  plate  and  allowed  to  darken 
by  exposure  to  the  air,  the  image  will  be  increased  in 
density  by  the  deposition  upon  it  of  the  coloured  products 
of  the  decomposition  of  the  pyrogallic  acid.  A  second 
negative  may  be  made  and  put  on  the  first,  and  if  this  is 
made  on  a  paper  or  film  support,  it  may  be  attached  to  the 
back  of  the  other  in  a  permanent  way.  The  back  of  the 
negative  may  be  coated  with  dextrine  made  sensitive  with 
a  bichromate,  and  then  exposed  to  light  through  the 
negative.  Where  the  light  acts,  the  stickiness  of  the  dex¬ 
trine  is  diminished  by  the  change  that  takes  place  in  it, 
and  if  then  a  pigment  in  powder  is  lightly  brushed  over 
it,  it  will  adhere  to  those  parts  where  the  light  has  not 
acted,  and  produce  a  kind  of  second  negative  on  the  back 
1  of  the  plate  which  will  reinforce  the  original  image.  All 
these  methods  have  been  used  and  some  of  them  exten- 

Isively,  and  some  of  them  may  still  be  of  value  in  certain 
;  difficult  cases. 

The  commonest  of  all  methods  of  altering  the  negative 


Finishing  the  Negative 

so  that  it  will  give  the  kind  of  print  that  is  desired,  so 
far  as  professional  portraitists  are  concerned,  goes  by 
the  name  of  tl  retouching.”  The  film  is  coated  with  a 
varnish,  put  on  by  gentle  friction,  that  gives  a  finely  matte 
surface,  so  that  marks  may  be  made  upon  it  with  a  lead 
pencil.  The  retoucher  then  strengthens  those  parts  that 
need  it  by  shading  them  with  the  pencil  in  a  suitable 
manner,  but  on  an  entirely  different  principle  from  the 
methods  described  above.  He  does  not  seek  to  maintain 
the  character  of  the  negative,  but  rather  to  alter  it.  His 
business  is  to  make  up  for  the  shortcomings  of  the 
photographer,  and,  as  a  rule,  also  to  alter  the  negative  so 
that  the  prints  may  be  more  acceptable  to  the  purchaser. 
Freckles,  spots,  and  wrinkles  that  are  too  pronounced  are 
modified  or  obliterated,  and  if  the  sitter  thinks  that  in  the 
photograph  her  cheek  is  too  round  or  her  waist  too  large, 
or  that  any  feature  is  not  shown  at  its  best,  it  is  the  re¬ 
toucher’s  duty  to  remedy  these  details.  If  a  part  of  the 
negative  is  too  dense,  so  that  it  gives  too  light  an  effect  in 
the  print,  he  will  scrape  or  pare  away  the  film.  And  by 
such  methods  he  will  alter  the  negative  in  any  desired  way 
to  please  his  customer. 

Retouching,  of  course,  is  not  photography,  and  a 
portrait  that  has  been  produced  by  this  combination  of 
processes  is  called  a  photograph  by  courtesy  rather  than 
in  fact.  A  few  of  the  most  notable  portrait  photographers 
do  not  retouch  their  portraits,  and  their  productions  are 
real  photographs.  From  a  business  point  of  view  there  is 
nothing  to  be  said  against  retouching,  but  a  retouched 
portrait  loses  much  if  not  all  of  its  value  as  evidence  of  the 
character  of  the  person  represented. 

In  scientific  work  of  all  kinds,  retouching  should  be 
strictly  avoided.  The  chief  value  of  a  photograph  is  that  it 
is  an  impersonal  record,  whether  of  a  person,  or  an  animal, 
or  a  building,  or  of  the  course  of  a  scientific  experiment. 
Any  handwork  on  either  the  negative  or  the  print  rendeis 

176 


/ 


Finishing  the  Negative 

the  photograph  unreliable.  The  manipulator  himself  may 
know  exactly  to  what  extent  he  has  modified  it,  but  even 
he  can  hardly  be  sure  that  his  handwork  has  not  extended 
to  some  little  detail  that  may  eventually  prove  to  be  of 
vital  consequence,  though  at  first  it  seemed  to  be  quite 
unimportant. 


1 77 


M 


CHAPTER  XI 

PRINTING  IN  SILVER 

The  chief  difference  between  a  photographic  print  and 
a  negative  is  the  reversal  of  the  lights  and  shades,  the 
opaque  or  dark  part  of  the  one  corresponding  to  the 
transparent  or  light  part  of  the  other.  In  the  very  early 
days  of  photography  this  was  the  only  distinction,  and 
even  at  the  present  time  what  are  generally  understood 
as  printing  methods  are  sometimes  used  for  the  prepara¬ 
tion  of  negatives,  and  plates  intended  for  negative  making 

are  sometimes  used  for  prints. 

As  soon  as  Fox  Talbot  found  that  a  greatly  enhanced 
sensitiveness  might  be  realised  by  the  development  of  an 
invisible  image  produced  by  the  exposure,  the  perfection 
of  photographic  processes  in  the  different  directions  of  the 
negative  and  the  print  was  begun.  The  great  sensitiveness 
that  is  so  valuable  in  shortening  the  necessary  exposure 
when  taking  a  negative  of  objects  that  move  or  are  likely 
to  move,  may  be  an  actual  drawback  in  a  printing  process 
because  of  the  difficulty  of  keeping  the  sensitive  surface 
absolutely  free  from  any  deposit  where  it  is  protected 
from  the  light.  And  in  a  print  the  colour  of  the  image  is 
of  fundamental  importance,  while  the  deposit  in  a  negative, 
though  preferably  black  or  grey,  that  it  may  affect  all 
the  constituents  of  light  equally,  may  be,  for  example,  of 
a  rather  dirty  greenish  shade  of  black  without  appreciable 
detriment,  a  colour  that  in  a  print  would  be  highly 
objectionable. 

The  form  of  silver  printing  that  most  resembles  tne 

178 


Printing  in  Silver 

making  of  negatives  is  called  “  bromide  ”  printing,  and 
the  paper  used  “  bromide "  paper,  the  word  u  bromide  ” 
being  a  contraction  of  the  expression  bromide  of  silver, 
taking,  as  is  usual  in  photography,  the  least  important 
word  or  part  of  a  word  to  represent  the  whole.  Bromide 
paper  is  paper  prepared  with  a  suitable  surface  and  then 
coated  with  a  bromide  of  silver  gelatine  emulsion.  It  is 
therefore  comparable  to  ordinary  gelatine  dry  plates,  but 
the  emulsion  is  not  made  so  sensitive  as  is  usual  for 
negative  work.  Nor  is  so  much  emulsion  put  upon  the 
paper ;  it  is  unnecessary  because  the  image  or  deposit 
produced  does  its  duty  twice  over.  In  a  negative  or 
transparency  the  deposit  has  to  be  dense  enough  to 
produce  its  effect  in  regulating  the  light  by  a  single 
passage  of  the  light  through  it,  but  on  a  paper  print,  the 
light  that  illuminates  it  passes  through  the  deposit  to 
the  white  paper  beneath,  and  is  then  reflected  from  the 
white  surface  and  passes  a  second  time  through  the 
deposit  that  forms  the  image,  and  so  to  the  eye.  If  the 
deposit  is  of  such  a  density  that  it  stops  four-fifths  of  the 
|  lights,  then  the  one-fifth  that  can  penetrate  to  the  paper 
will  suffer  a  like  diminution  in  its  passage  back  again,  and 
only  one-twenty-fifth  of  the  light  that  would  have  appeared 
if  there  had  been  no  deposit  there  will  survive  the  treat¬ 
ment.  Now  when  the  light  reflected  from  white  paper 
is  reduced  to  one-twenty-fifth  of  its  brightness,  that  part 
of  the  white  paper  appears  black.  The  blackness  is  not 
perhaps  of  the  very  inkiest  blackness  possible,  but  it  is  very 
near  to  it,  and  not  many  photographs  have  their  darkest 
1  parts  so  black  as  this.  But  a  deposit  of  silver  in  a  negative 
;  that  allows  one-fifth  of  the  light  to  pass  through  would 
be  very  inadequate  indeed  as  a  maximum  density.  In 
j  a  negative  we  should  be  able  to  get  a  deposit  that  will 
allow  only  about  one-hundredth  of  the  light  to  pass,  and 
sometimes  even  less  than  that. 

The  enhanced  effect  of  the  second  passage  through 

179 


Printing  in  Silver 


a  substance  that  impedes  the  passage  of  light,  may  be  seen 
by  lightly  shading  with  a  lead  pencil  a  small  patch  o 
tracing  paper.  The  difference  in  the  darkness  of  tie 
shading  when  it  is  held  up  to  the  window  and  looked 
through,  and  when  it  is  pressed  down  upon  a  piece so 
white  paper  and  looked  on,  will  clearly  illustrate  this 
matter.  These  considerations  with  regard  to  the  densi  y 
of  the  deposit  apply  to  all  printing  methods  on  paper 
or  opaque  surfaces,  and  show  why  it  is  often  difficult 
and  may  be  impossible  to  make  a  useful  negative  on 
a  paper  prepared  for  printing  purposes.  Paper  that  is 
specially  intended  for  negative  work  has  a  thicker  coating 
of  emulsion  put  upon  it.  To  obtain  a  iisefu  nega  iv 
instead  of  a  print,  it  is  necessary  to  considerably  increase 
the  duration  of  both  the  exposure  and  the  development, 
that  the  image  may  be  of  the  requisite  densi  y.  n  e 
these  conditions  printing  papers  of  suitable  quality  y  s 
sometimes  be  advantageously  used  for  the  production 
duplicate  and  especially  enlarged  negatives.  > 

In  the  making  of  bromide  prints  the  paper  is  exposed 
under  the  negative  for  a  few  seconds  to  a  gas  flame  or 
other  light  of  moderate  intensity,  or  if  an  enlargement 
is  being  made,  the  image  produced  by  the  enlarging 
lantern  or  camera  is  allowed  to  fall  upon  it  or  a  s  o 
time,  the  paper  is  developed,  fixed,  washed  and  dried 
The  developer  must  be  of  such  a  nature  that  s  ami  g  I 
the  paper  is  absolutely  avoided,  and  a  little  allowance  has 
to  be  made  in  developing  the  image  for  the  increase  of 
its  blackness  by  the  fixing.  1  his  increase  is  due 
removal  of  the  white  particles  of  silver  bromide  wh  ch 
before  fixing  are  interspersed  among  and  underlie  the 
black  particles  of  the  silver  and  give  the  image  a  greyish 

To  avoid  the  production  of  an  unpleasant  greenish 
black  image,  it  is  necessary  to  use  only  the  minimum  of 
potassium  bromide  as  restr'ainer  in  the  developer. 


Printing  in  Silver 

less  various  developers  give  slightly  different  results,  so  far 
as  a  slight  tinge  of  colour  in  the  blackness  of  the  image 
is  concerned,  but  there  is  reason  to  suppose  that  these 
differences  have  been  often  stated  to  be  very  much  greater 
than  they  really  are.  Organic  developers  are  now  gener¬ 
ally  used,  though  a  little  while  ago  there  was  a  prejudice 
in  favour  of  ferrous  oxalate.  But  the  limitation  with 
regard  to  potassium  bromide  applies  to  them  all,  and  it 
is  therefore  not  possible  to  use  such  a  restrainer  in  order 
to  adjust  the  gradation  of  the  print.  It  is  possible,  how¬ 
ever,  to  reduce  the  contrast  in  a  print  by  increasing  the 
exposure,  making  the  developer  work  more  slowly  by 
adding  water  to  it,  and  stopping  the  development  before 
it  is  complete.  The  image  will  then  be  of  a  warmer  black 
than  if  development  had  been  allowed  to  proceed  further, 
and  it  follows  therefore  that  the  tint  of  the  image  depends 
upon  the  relationship  that  the  gradation  of  the  print 
bears  to  the  gradation  of  the  negative.  The  reddish  tinge 
is  presumably  due  to  some  of  the  particles  which  form 
the  image  being  smaller  than  .when  the  fully  black  image 
is  obtained.  But  this  redness,  even  when  the  exposure 
has  been  increased  to  four  or  eight  times,  is  hardly  ob¬ 
servable,  and  the  print  appears  to  be  of  a  good  black 
unless  a  colder  (or  less  red  or  bluer)  black  is  put  by  the 
side  of  it  and  the  two  are  compared. 

The  difference  in  colour  obtainable  by  the  variation 
of  exposure  and  development  within  practical  limits  is 
very  slight,  and  depends  largely  on  the  character  of  the 
negative.  Those  who  prefer  a  brown  to  a  black  image 
must  therefore  generally  alter  the  colour  after  the  print 
has  been  otherwise  finished.  A  very  favourite  method 
of  doing  this  is  to  cause  sulphur  to  combine  with  the 
metallic  silver  or  some  of  it.  This  method  was  introduced 
and  popularised  by  the  Eastman  Kodak  Company,  who 
recommended  heating  the  print  in  a  solution  containing 
alum  and  sodium  hyposulphite.  Such  a  mixture  deposits 

181 


Printing  in  Silver 

sulphur  copiously,  and  by  its  decomposition  unstable  com¬ 
pounds  are  produced  which  will  slowly  continue  to  give 
up  sulphur  to  the  silver.  The  method  is  slow,  requiring 
many  hours  if  the  liquid  is  at  the  ordinary  air  temperature, 

and  perhaps  half  an  hour  if  heated. 

Under  these  or  any  other  practical  conditions,  silver 
combines  very  slowly  with  sulphur.  It  is  therefore  quicker, 
and  to  a  certain  extent  a  more  controllable  method,  to 
add  some  other  substance,  such  as  chlorine  or  bromine, 
to  the  silver,  and  then  to  exchange  this  added  substance 
for  sulphur.  This  is  the  form  of  the  sulphur  method 
of  toning  that  is  at  present  generally  preferred.  If  the 
substance  that  supplies  the  chlorine  or  its  equivalent  be¬ 
comes  insoluble  by  the  loss  of  the  chlorine  that  it  gives 
up,  then  the  residue  of  the  reagent  employed  remains  with 
the  silver  compound  that  is  formed.  By  this  and  analogous 
methods,  compounds  of  many  metals  may  be  added  to  the 
original  silver  image,  metals  such  as  iron,  copper,  uranium, 
vanadium,  and  antimony,  and  by  acting  on  these,  an 
image  of  almost  any  colour  that  may  be  desired  can 
be  obtained.  Brown,  blue,  green,  red,  yellow,  and  in¬ 
termediate  and  various  shades  of  these  colours  are  easily 
produced,  and  it  is  customary  to  describe  such  prints 
as  toned  bromide  prints.  But  it  is  obvious  that  by  such 
manipulation  the  character  of  the  print  has  been  vitally 
altered.  The  material  of  the  image  is  no  longer  metallic 
silver,  indeed  it  may  have  no  silver  or  silver  compound 
left  in  it,  but  it  consists  of  more  or  less  complex  mixtures 
of  substances,  such  as  Prussian  blue,  the  sulphides,  ferro- 
cyanides,  chromates  or  other  compounds  of  the  metal  or 
metals  that  may  have  been  used.  A  pure  silver  image 
may  last  without  perceptible  change  for  twenty  years  or 
more,  but  a  “  toned  bromide  ”  print  may  deteriorate  within 
twenty  hours.  Such  prints  prepared  for  exhibition  have 
been  known  to  change  within  a  week  or  two  to  a  notable 
degree  as  they  hung  upon  the  wall.  A  print  ought  to 


Printing  in  Silver 


be  described  as  what  it  is,  and  not  as  what  it  was,  but 
an  attempt  to  do  this  in  such  cases  would  introduce 
difficulties,  as  there  are  no  common  names  for  many  of 
the  substances  produced  in  toning,  and  some  of  the 
changes  are  of  such  a  mixed  character  that  it  is  impossible 

to  say  what  remains  in  the  film. 

There  is  another  kind  of  silver  printing  paper,  though 
similar  in  many  ways  to  bromide  paper,  the  different 
varieties  of  which  are  commonly  called  “gas  light  papers. 
These  generally,  if  not  always,  contain  chloride  of  silver 
mixed  with  the  bromide  of  silver,  and  are  called  “gas 
light”  because  they  are  so  little  sensitive  that  they  may 
be  cut  up,  put  in  the  printing  frames,  and  otherwise  mani¬ 
pulated  as  necessary,  in  a  room  lit  by  ordinary  gas  light, 
with  no  other  precaution  than  not  allowing  the  gas  light 
to  shine  directly  upon  them.  The  exposure  under  the 
negative  may  be  to  an  ordinary  gas  flame  for  from  one 
to  about  five  minutes  or  longer  at  a  distance  of  about 
twelve  inches.  The  development  may  be  carried  out  in 
a  shaded  part  of  the  same  room,  or  sometimes  it  is 
convenient  to  protect  the  paper  from  too  much  light 
during  its  manipulation,  by  seeing  that  one’s  own  shadow 
always  falls  upon  it.  A  chloro-bromide  emulsion  will  give 
an  image  inclining  to  redness  much  more  easily  than  a 
simple  bromide  emulsion  ;  indeed  it  is  possible  with  some 
to  get  a  fully  red  or  “  red  chalk  ”  colour,  by  simply  pro¬ 
longing  the  exposure  and  developing  with  a  developer 
that  is  very  strongly  and  suitably  restrained.  A  con¬ 
venient  method  of  exposure  in  such  cases  is  by  the 
burning  of  magnesium  ribbon,  and  it  may  be  necessary 
to  use  as  much  as  six  inches  or  more  and  to  burn  it  at 
a  distance  of  only  five  or  six  inches  from  the  negative. 
But  this  method  of  modifying  the  colour  is  not  advan¬ 
tageous  beyond  very  narrow’  limits,  because  the  greatly  in¬ 
creased  exposure  tends  to  give  a  flat  print,  and  the  vigour 
of  the  image  is  so  much  reduced  by  the  fixing  bath  that 

183 


Printing  in  Silver 

it  is  difficult  to  know  how  far  to  develop  to  get  the  desired 
final  result.  The  image  in  prints  of  this  kind,  being 
produced  by  development,  consists  of  particles  of  metallic 
silver,  and  is  therefore  as  amenable  to  after  processes, 
such  as  toning,  as  the  image  on  negatives  and  ordinary 
bromide  paper. 

The  most  common  of  all  the  methods  of  silver  printing, 
although  perhaps  the  most  simple  so  far  as  manipulation 
is  concerned,  is  the  most  complex  from  a  chemical  point 
of  view.  There  is  no  development,  the  image  is  produced 
in  its  full  intensity  by  simple  exposure  to  light,  and  the 
process  has  for  the  last  twenty  years  been  called  “  printing- 
out,”  and  the  paper  suitable  for  it  “  printing-out-papers ' 
or  “P.O.P.”  All  photography  was  of  the  “printing-out” 
kind  until  the  possibility  of  developing  an  invisible  image 
was  discovered.  In  these  processes  chloride  of  silver 
is  used  without  any  bromide,  but  it  cannot  be  used  alone, 
because  the  action  of  light  upon  chloride  of  silver  is  to 
cause  some  of  the  chlorine  to  separate  from  the  com¬ 
pound,  and  this  decomposition  will  not  take  place  if  the 
chloride  of  silver  is  absolutely  pure  and  dry.  There  must 
be  something  present  to  which  the  chlorine  can  attach 
itself  or  it  will  not  leave  the  silver  salt,  and  if  this 
“sensitiser,”  as  it  is  called,  is  not  sufficiently  powerful, 
the  chlorine  that  accumulates  in  it  will  act  in  the  opposite 
direction  to  the  light,  and  prevent  the  production  of  a 
fully  coloured  and  vigorous  image.  But  the  presence  of 
the  sensitiser  often  leads  to  something  more  than  just 
the  simple  change  described,  it  produces  other  silver 
compounds,  and  these  as  well  as  the  chloride  of  silver 
have  to  be  taken  into  account.  Thus  the  changes  are 
mixed  and  complicated,  and  in  no  practical  method  have 
they  been  fully  followed  out.  And  when  we  add  that 
the  image  as  produced  by  simple  exposure  and  fixing 
is  an  unpleasant  red  and  has  to  be  toned  by  the  action 
of  other  substances  upon  it,  it  is  not  surprising  that  the 


Printing  in  Silver 

material  of  which  the  image  finally  consists  is  a  mixture 
of  unknown  products.  Although  these  printing-out 
silver  processes  can  never  merit  the  confidence  that 
some  other  methods  deserve,  there  has  been  such  a  vast 
experience  with  them,  that  from  a  practical  point  of 
view,  they  are  not  much  inferior  to  developed  silver 
prints,  assuming  proper  care  to  be  exercised  in  their 
production. 

The  earliest  printing-out  methods  were  “plain  paper” 
processes,  that  is,  there  was  no  film  on  the  paper,  and 
for  the  sake  of  a  brilliant  image  as  distinguished  from 
a  “  sunk  in  ”  appearance,  the  endeavour  was  made  to 
keep  the  sensitive  compounds  on  the  surface  rather 
than  allow  them  to  sink  into  the  body  of  the  paper. 
The  sizing  of  the  paper  was  found  to  be  a  very  important 
factor  in  the  matter,  because  it  is  this  that  confers  the 
non-absorbent  property  and  makes  the  difference  between 
blotting  paper  and  writing  or  printing  paper.  So  some- 
1  times  a  little  sizing  material  such  as  gelatine  or  starch 
;  was  added  to  the  mixture  used  before  the  paper  was 
coated  with  it.  It  was  but  a  step  from  this  to  the  use 
|  of  albumen,  used  in  the  form  of  white  of  egg,  and  the 
getting  of  a  definite  film  on  the  paper.  It  is  not  certainly 
known  by  whom  or  when  albumen  was  first  used,  but 
both  albumen  and  gelatine  were  used  before  1851,  and 
the  earliest  recorded  use  of  albumen  appears  to  date 
from  about  1848. 

The  use  of  albumen  is  worthy  of  note,  because  for 
:  thirty  or  forty  years  albumenised  paper  was  the  silver 
printing  paper,  and  the  expression  “silver  print”  meant 
I  a  print  on  albumenised  paper,  and  the  word  “photograph  ” 
in  common  language  meant  the  same  thing.  The  peculiar 
colours  of  the  image  obtained  when  such  prints  were 
toned  in  the  ordinary  way  were  known  as  “photographic 
purple,”  &c.,  and  people  so  thoroughly  associated  these 
^colours  with  “  photographs,”  that  when  other  printing 


Printing  in  Silver 

methods  were  introduced  it  was  necessary  to  imitate 
these  “  photographic  ”  colours  in  order  to  make  people 
believe  that  the  prints  were  photographs  and  to  get 
them  appreciated  as  such. 

Albumenised  silver  paper  was  prepared  by  adding  a 
little  ammonium  chloride  to  the  whites  of  eggs,  beating 
up  well  so  that  the  animal  membrane  might  be  removed, 
and  filtering  the  mixture  through  muslin  or  flannel  into 
a  flat  dish  so  that  a  sufficient  surface  of  it  might  be 
available.  Each  sheet  of  paper  was  gradually  lowered 
upon  the  preparation  until  it  floated  on  it,  and  then 
gradually  raised  and  hung  up  to  dry.  To  sensitise  the 
paper  it  was  floated  on  a  rather  strong  solution  of  silver 
nitrate,  and  after  a  minute  or  two  removed  and  hung  up 
to  dry.  The  silver  nitrate  coagulates  the  albumen  or 
renders  it  insoluble  in  water  in  much  the  same  way 
that  11  boiling "  an  egg  affects  the  white  of  the  egg,  so 
that  there  is  no  fear  of  the  albumen  being  dissolved  off 
the  paper  in  its  subsequent  treatment.  It  forms  silver 
chloride  and  ammonium  nitrate  with  the  ammonium 
chloride,  it  produces  a  compound  of  albumen  containing 
silver,  so  that  all  these  things  with  an  excess  of  silver 
nitrate  remain  on  the  paper.  What  chemical  changes 
take  place  during  the  exposure  are  not  known,  but 
probably  the  chief  part  played  by  the  silver  nitrate  is 
to  facilitate  the  decomposition  of  the  other  silver  com¬ 
pounds  into  coloured  substances.  A  print  made  on  such 
paper  and  fixed  in  hyposulphite  of  soda  would  show  an 
image  of  a  disagreeable  red  colour,  and  in  order  to  get 
a  more  agreeable  purple,  brown,  sepia,  or  an  approxima¬ 
tion  to  black,  it  was  usual  to  tone  it  by  putting  it  into 
a  suitable  solution  containing  gold,  though  sometimes 
platinum  and  other  metals  were  used.  By  far  the  greater 
number  of  photographs  and  all  the  small  portraits  of  a 
generation  or  so  ago  were  made  by  this  method.  Some 
lasted  ten,  twenty,  or  even  thirty  years  without  obvious 


Printing  in  Silver 

change,  but  the  greater  number  showed  signs  of  fading 
within  a  shorter  period ;  and  if  carelessly  prepared  or 
not  properly  washed,  or  pasted  upon  an  inferior  mount, 
or  pasted  with  sour  paste,  or  otherwise  subjected  to 
adverse  influences,  they  might  last  no  more  than  a  few 
months  before  showing  signs  of  change. 

As  albumenised  paper  became  more  largely  used,  it 
was  prepared  commercially  ready  for  sensitising,  and 
eventually  it  was  sold  ready  sensitised,  and  as  when 
sensitised  it  would  not  keep  in  good  condition  for  more 
than  a  day  or  two,  preservatives,  generally  citric  acid, 
were  employed.  But  when  the  demand  for  printing 
papers  increased,  it  was  obvious  that  the  manipulation 
of  single  sheets  of  paper  was  disadvantageous,  and  that 
the  method  to  aim  at  was  the  preparation  in  bulk  of 
an  emulsion  that  could  be  coated  upon  the  paper  by 
machinery.  This  led  to  the  introduction  of  gelatino- 
chloride  and  collodio-chloride  printing  papers,  and  as 
these  would  keep  in  good  condition  longer  than  sensitised 
albumen  paper,  and  gave  results  that  were  generally 
considered  richer  in  colour  and  in  general  appearance, 
everything  was  in  their  favour.  These  papers  were 
introduced  between  1885  and  1890  in  America,  and  in 
England  in  1891  by  the  Ilford  Company.  Other  firms 
soon  arranged  for  the  manufacture  of  them,  and  albu¬ 
menised  silver  paper  gradually  became  a  thing  of  the 
past.  It  is  papers  of  this  type  that  are  now  referred  to 
by  the  generic  title  of  printing-out-papers  or  P.O.P. 

Printing-out-papers  are  known  by  all  sorts  of  fancy 
names,  and  presumably  are  prepared  by  many  different 
formulae,  but  the  general  method  of  preparation  is  to 
introduce  into  the  medium  a  chloride,  nitrate  of  silver 
(which  forms  chloride  of  silver  by  reacting  with  the 
chloride ;  but  more  nitrate  of  silver  is  added  than  is 
sufficient  to  complete  this  change,  so  that  there  remains 
some  unchanged  silver  nitrate),  an  organic  salt  such  as 

187 


Printing  in  Silver 

a  citrate  or  a  tartrate  (to  give  more  body  to  the  image), 
and  citric  acid  which  preserves  the  sensitive  salts  from  a 
too  rapid  spontaneous  change.  This  emulsion  is  spiead 
upon  paper  that  has  already  been  prepared  by  coating 
it  with  a  suitable  substance  that  gives  it  a  smooth  and 
slightly  absorbent  surface. 

When  such  paper  is  exposed  under  a  negative  until 
the  image  is  somewhat  darker  than  is  desired  for  it 
loses  in  depth  in  the  after-treatment  and  then  fixed,  the 
colour  will  be  of  an  unpleasant  reddish  colour,  much  as 
in  the  case  of  albumenised  paper.  Hence  the  need  for 
toning.  The  chloride  of  gold  used  for  this  purpose  is 
always  mixed  in  solution  with  another  substance,  pre¬ 
ferably  ammonium  sulphocyanide,  and  when  the  washed 
print  is  put  into  the  solution,  gold  is  deposited  upon 
the  image  in  a  form  which  appears  blue,  and  the  red  of 
the  image  is  thus  modified  to  the  rich  nondescript 
colours  that  are  so  familiar.  1  he  print  is  then  fixed  in 
hyposulphite  of  soda  to  remove  all  the  silver  compounds 
that  are  sensitive  to  light,  and  well  washed. 

Gold  may  be  deposited  by  such  means  as  just  described 
in  either  a  bluish  or  a  reddish  form,  and  it  appears  that 
the  difference  in  colour  is  due  to  a  difference  in  the  size 
of  the  particles  in  which  it  is  deposited.  In  toning  it  is 
the  bluish  form  only  that  is  effective,  and  the  rapidity 
of  action  of  the  toning  bath  is  arranged  in  order  to 
secure  this.  But  as  the  change  of  colour  seems  to  be 
due  to  the  deposition  of  particles  of  a  certain  size  rather 
than  of  a  certain  substance,  other  substances  besides  gold 
might  be  expected  to  serve,  and  so  indeed  they  will.  It 
appears  that  the  earliest  toning  baths  used,  about  sixty 
years  ago,  did  not  contain  gold  at  all,  or  at  least  only 
as  an  alternative,  the  change  of  colour  being  brought 
about  by  adding  to  the  fixing  bath  some  substance  that 
would  cause  the  deposition  of  sulphur  from  it,  so  that 
toning,  by  the  deposition  of  sulphur,  and  fixing  went  on 

188 


Stonehenge 

AS  SEEN  FROM  AN  AEROPLANE. 

Photographs  from  balloons  and  sometimes  from  kites  have  been  made  since 
many  years  ago  for  military  surveying  purposes.  Whether  or  not  aeroplanes  will 
prove  more  advantageous  for  such  work  remains  to  be  seen. 


Printing  in  Silver 

simultaneously.  A  little  acid  added  to  the  fixing  bath 
will  do  this  ;  but  simple  acid  is  not  advisable  ;  and  it  was 
usual  to  add  more  than  one  thing  and  sometimes  many 
— ferric  chloride  and  silver  nitrate,  or  iodine  and  silver 
nitrate,  with  the  addition  of  lead  nitrate,  or  gold  chloride 
and  silver  nitrate.  Such  baths  are  called  “combined,” 
because  they  serve  the  double  purpose  of  toning  and 
fixing.  Even  to  the  present  day  it  is  usual  to  give 
formulae  for  combined  baths,  to  draw  attention  to  the 
rich  colours  that  they  give,  and  to  warn  photographers 
not  to  use  them.  Judging  merely  by  the  colour  that 
they  give,  these  baths  improve  by  use,  and  they  continue 
to  give  satisfactory  results  to  the  eye  after  probably  all 
the  gold  has  been  removed  from  them.  It  is  impossible 
to  follow  all  the  changes  that  may  take  place,  but  they 
are  certainly  many  more  than  when  the  toning  and  fixing 
solutions  are  kept  separate.  The  resulting  image,  though 
it  may  be  of  a  pleasant  colour,  is  often  very  unstable. 

The  self-toning  papers  now  so  common  have  the 
necessary  gold  salt  added  to  the  emulsion.  Some  of  these 
are  put  in  a  weak  solution  of  common  salt  before  fixing 
to  allow  the  toning  to  take  place,  others  may  be  fixed 
straight  away.  Although  the  gold  salt  in  the  latter  case 
goes  into  the  hyposulphite  solution  and  so  forms  a  kind 
of  combined  bath,  the  self-toning  paper  has  the  advantage 
that  every  piece  of  paper  takes  into  the  fixing  bath  the  gold 
necessary  for  its  toning,  so  that  the  bath  cannot  be  used 
after  all  its  gold  has  been  removed  from  it,  a  very  likely 
occurrence  with  the  ordinary  combined  bath.  There  can 
be  no  doubt  that  the  safest  course  is  to  keep  the  toning  and 
fixing  quite  separate,  for  then  the  possible  changes  are 
simplified,  and  if  the  toning  bath  is  too  weak  or  is  getting 
exhausted  its  slower  or  incomplete  action  is  shown  at 
once. 

Although  the  sensitive  papers  now  under  consideration 
are  printing-out-papers,  it  is  possible  to  partly  print  out 

189 


Printing  in  Silver 

and  then  to  complete  the  production  of  the  image  by 
development  with  an  alkaline  developer.  The  Paget  Prize 
Plate  Company,  who  introduced  this  method  of  work  in 
1893,  say  that  “  the  actual  time  of  -exposure  may  be  any¬ 
thing  from  that  necessary  to  fully  print  out,  down  to  that 
only  sufficient  to  produce  a  faint  image  of  the  stronger 
details.”  If  the  developer  were  applied  to  such  a  print, 
the  image  would  be  at  once  buried  in  blackness.  It  is 
necessary  to  soak  the  print  before  development  in  a  rather 
strong  solution  of  potassium  bromide,  in  order  to  convert 
the  silver  salts  as  far  as  possible  into  bromide,  and  also 
to  use  a  large  proportion  of  potassium  bromide  in  the 
developer.  The  developed  print  then  needs  to  be  toned 
and  fixed  as  if  it  had  been  produced  by  a  full  exposure 
without  development. 

There  is  a  curious  method  of  silver  printing,  or  rather 
of  using  silver  paper  for  getting  first  a  negative  and  from  it 
a  print,  that  is  generally  called  “  Playertype  ”  after  Mr.  J. 
Hort  Player  who  described  the  method  in  1896.  Mr. 
Player  did  much  to  perfect  the  method,  and  produced  some 
excellent  results  by  its  means.  The  strange  detail  in  the 
process  is  that  the  sensitive  paper  is  put  face  downwards 
on  to  the  original  of  which  a  photographic  copy  is  de¬ 
sired,  and  then  exposed  so  that  the  light  passes  through 
the  sensitive  paper  before  it  gets  to  the  original.  It  might 
be  thought  that  the  paper  would  be  completely  fogged  as 
there  is  nothing  over  it  to  regulate  the  action  of  the  light, 
and  so  indeed  it  is,  but  not  to  such  an  extent  as  to  render 
useless  the  image  that  is  produced  by  the  light  reflected 
from  the  surface  ^>f  the  original.  By  this  means  a  negative 
results,  and  from  this  a  print  is  produced  by  exposure  of 
the  paper  under  the  negative  in  the  usual  way.  It  is 
obvious  that  the  copy  can  only  be  of  the  same  size  as  the 
original.  The  special  advantage  of  the  method  is  that 
illustrations  or  letterpress  in  books,  pencil  drawings, 
photographic  prints,  or  anything  of  the  sort  can  be  copied 

190 


Printing  in  Silver 

even  if  on  opaque  mounts,  and  without  interference  from 
any  printing  or  device  that  may  be  on  the  back  of  them. 

The  method  that  Mr.  Player  found  to  be  the  most 
successful,  was  to  place  a  piece  of  gelatino-chloride  paper 
face  downwards  on  the  engraving  to  be  copied  ;  on  the  top 
a  yellow  screen  was  put  with  an  extra  sheet  of  glass  if 
necessary  to  press  the  whole  into  close  contact  by  its 
weight,  and  exposure  was  made  to  daylight.  With  a  green 
screen  gas-light  might  be  used.  Hydroquinone  was  used 
as  developer  with  a  little  potassium  iodide  added  to  it. 
All  these  details  are  such  as  would  tend  to  increase  the 
contrast  in  the  negative  produced,  and  so  to  get  vigorous 
detail  in  spite  of  the  general  fog.  That  such  a  process 
should  be  practically  possible,  seems  to  indicate  that  a 
considerable  amount  of  light  may  pass  through  a  sensitive 
film  without  affecting  it,  although  the  light  that  has  passed 
through  is  well  able  to  produce  its  characteristic  result 
when  it  is  sent,  by  reflection  from  the  surface  beneath, 
through  the  film  a  second  time.  That  this  really  is  so  may 
be  confirmed  in  other  ways. 


CHAPTER  XII 

OTHER  PRINTING  METHODS 

It  is  possible,  directly  or  indirectly,  to  get  photographic 
prints  in  almost  any  material.  Some  processes  are  not 
worth  following  up  because  they  give  unpleasant  colours, 
others  because  they  are  too  costly  and  offer  no  com¬ 
mensurate  advantage,  and  others  because  they  are  too 
tedious  or  the  result  is  too  unstable,  or  for  some  such 
practical  reason. 

Silver  salts  were  the  earliest  sensitive  compounds  em¬ 
ployed,  and  they  have  always  retained  the  foremost  posi¬ 
tion,  partly  perhaps  because  they  furnish  processes  that 
are  easy  to  work  by  those  who  are  able  to  make  negatives, 
and  partly  because  the  general  public  has  got  used  to  them. 
The  colours  of  the  image  that  they  give  are  often  peculiar 
to  themselves,  but  they  are  so  much  appreciated  that 
other  methods  which  naturally  would  give  colours  more 
welcome  to  educated  eyes,  have  had  to  imitate  the  appear¬ 
ance  of  silver  prints  in  order  to  gain  a  measure  of  patron¬ 
age.  The  chief  disadvantage  that  all  silver  prints  suffer 
from  is  their  want  of  stability.  In  this  they  vary  much 
according  to  the  method  of  their  production  and  the  care 
exercised  in  the  preparation  of  them.  Some  will  last  for 
a  year  or  two,  some  for  a  decade  or  two,  and  a  very  small 
proportion  may  be  in  fair  condition  after  a  generation. 
Although  imperfect  fixing  and  washing  certainly  tend  to 
shorten  the  life  of  a  silver  print,  it  is  impossible  to  say 
definitely  why  there  should  be  so  much  variation  in  the 
permanence  of  prints  produced  apparently  with  due  care. 
The  fact  is  that  finely  divided  metallic  silver  and  silver 

192 


Other  Printing  Methods 

compounds  are  susceptible  to  the  action  of  many  adverse 
conditions,  and  it  is  not  always  possible  to  guard  against 
these.  The  superior  stability  of  negatives  is  occasionally 
mentioned  as  evidence  that  silver  prints  might  be  more 
permanent,  but  the  two  cases  are  very  different.  In  the 
negative  there  is  a  much  greater  quantity  of  silver,  and  it 
is  more  thoroughly  embedded  in  the  gelatine  and  therefore 
better  protected  from  outside  influences,  and  on  the  glass 
side  its  protection  is  perfect.  The  print  is  generally 
mounted,  and  the  paste  used  as  well  as  the  card  may 
contain  deleterious  matter.  Therefore  all  the  chances  of 
a  long  life  are  in  favour  of  the  negative. 

The  only  way  to  ensure  permanency  is  to  give  up  silver 
altogether  so  far  as  the  print  is  concerned.  Platinum  and 
carbon  are  two  substances  that  are  as  little  liable  to  change 
as  any  material,  and  both  of  these  are  practically  avail¬ 
able  and  give  their  names  to  processes  that  are  extensively 
used  at  the  present  time.  They  can,  however,  only  be 
employed  indirectly,  for  no  platinum  or  carbon  compound 
is  usefully  sensitive  to  light. 

Platinum,  from  its  inertness,  seemed  such  an  advan¬ 
tageous  material  to  use  for  the  image  of  the  print,  that  we 
find  attempts  to  formulate  a  method  for  it  as  far  back  as 
1845.  These  early  experiments  failed  because  the  com¬ 
pound  of  platinum  with  the  maximum  amount  of  chlorine 
was  used,  so  that  even  if  some  of  its  chlorine  was  removed 
directly  or  indirectly  by  the  agency  of  light,  the  soluble 
compound  with  less  chlorine  in  it  was  produced,  and  the 
amount  of  metal  deposited  was  very  slight,  if  any.  Some 
of  them  failed  because  it  was  sought  to  get  a  sensitive 
platinum  compound,  and  to  get  the  visible  image  pro¬ 
duced  during  the  exposure.  The  conditions  necessary  for 
success  were  realised  by  Mr.  W.  Willis  in  1873,  and  by 
1880  he  had  succeeded  in  doing  without  the  addition  of 
any  silver  or  lead  compound,  which  till  then  he  had  found 
helpful. 


193 


N 


Other  Printing  Methods 

The  sensitive  substance  that  is  used  in  platinum  paper 
is  ferric  oxalate,  and  this  dissolved  in  water  with  the  plati¬ 
num  salt  forms  a  yellow  solution  which  is  applied  to  the 
surface  of  a  suitable  paper.  The  prepared  paper  is  yellow 
on  the  coated  side,  and  it  is  notable  that  the  ordinary 
papers  for  platinum  printing  have  no  film,  as  of  gelatine 
or  albumen,  on  the  surface  ;  the  substances  applied  are 
actually  on  the  paper  itself  and  penetrate  a  little  way  into 
it.  To  prevent  the  coating  solution  from  penetrating  too 
far,  the  paper  is  suitably  sized.  When  the  paper  is  ex¬ 
posed  to  light,  the  iron  salt,  that  is  the  ferric  oxalate,  is 
decomposed  so  that  a  part  of  the  substance  that  is  com¬ 
bined  with  the  iron  is  driven  off  from  it,  and  the  iron  in 
the  compound  that  remains,  ferrous  oxalate,  is  now  able 
under  suitable  conditions  to  combine  with  something  else. 
These  conditions  are  brought  about  when  the  exposed 
print  is  developed,  and  the  iron  compound,  being  in  the 
immediate  presence  of  the  platinum  salt,  takes  chlorine 
from  it  and  leaves  the  metal  platinum  as  a  grey  or  black 
deposit  to  form  the  image.  The  platinum  compound  used 
is  the  chloride  that  contains  the  least  possible  proportion 
of  chlorine,  so  that  no  chlorine  whatever  can  be  taken 
from  it  without  a  corresponding  deposition  of  the  metal. 
The  developed  print  is  put  into  weak  hydrochloiic  acid, 
so  that  all  the  substances  in  the  paper,  except  the  metallic 
platinum  which  is  quite  insoluble,  may  be  dissolved  out 
from  it;  the  acid  is  changed  a  few  times  that  its  action 
may  be  complete,  the  acid  is  washed  away  with  water,  and 
the  print  is  dried. 

By  varying  what  may  be  called  the  minor  conditions, 
paper  may  be  prepared  so  that  it  is  advantageously  de¬ 
veloped  on  either  a  hot  or  a  cold  solution  of  the  developing 
agent,  and  by  the  addition  of  a  very  small  proportion  of 
mercuric  chloride  to  the  solution  with  which  the  paper 
is  coated,  and  a  few  little  adjustments  in  other  details, 
the  metallic  platinum  will  be  deposited  so  that  a  reddish 

*94 


Other  Printing  Methods 

brown  or  “  sepia "  image  is  obtained  instead  of  black. 
This  coloured  deposit  consists  of  pure  platinum,  and  its 
colour  is  doubtless  due  to  the  particles  of  metal  being 
smaller  than  when  the  image  is  black. 

The  permanent  nature  of  the  platinum  image  can  be 
inferred  from  the  fact  that  no  method  has  yet  been  found 
of  altering  it  or  removing  a  part  of  it,  as,  for  example,  by 
dissolving  it.  Aqua  regia  will  dissolve  platinum,  but  if  a 

1  platinum  print  is  put  into  it,  the  paper  is  disintegrated 
and  destroyed  before  the  image  is  dissolved.  Although  it  is 
impossible  to  get  platinum  to  combine  with  anything  and 
so  to  tone  the  print  by  any  of  the  methods  that  work  so 
easily  with  silver  prints,  it  is  possible  to  deposit  other 
things  upon  the  platinum.  If  a  liquid  is  just  about  to 
deposit  a  constituent  that  it  has  in  solution  and  a  platinum 
print  is  put  into  it,  the  platinum  will  cause  the  deposition 

I  to  take  place  in  its  immediate  presence  a  little  sooner  than 
it  otherwise  would,  and  in  this  way  gold  which  is  bluish, 
uranium  ferrocyanide  which  is  a  reddish  brown,  and 
doubtless  many  other  things  can  be  added  to  the  platinum, 
and  so  the  colour  of  the  image  may  be  somewhat  changed. 

I  Any  substance  so  added  can  be  removed  again  if  there 
are  means  suitable  for  the  purpose,  and  the  platinum  will 
remain  unchanged. 

It  is  very  difficult  to  remove  the  last  small  portions 
of  the  iron  salts  which  remain  in  the  paper  after  develop¬ 
ment,  and  however  carefully  the  prints  are  washed  there 
j  will  be  a  minute  trace  of  iron  compound  that  cannot 
be  got  rid  of,  and  this  will  cling  especially  where  the 
!  platinum  has  been  deposited.  By  soaking  the  print  in 
j  a  solution  of  gallic  acid  or  an  extract  of  catechu  this 
1  ferruginous  residue  is  turned  to  a  brown  compound,  and 
this  imparts  a  warm  tone  to  the  print.  Some  platinum 
prints,  especially  those  that  have  not  been  very  carefully 
t  washed  or  have  been  mounted  on  inferior  cardboard,  have 
been  noticed  to  gradually  change  to  a  yellowish  brown 

195 


Other  Printing  Methods 

colour,  that  really  appears  as  if  the  print  were  fading 
like  a  silver  print.  But  this  change  appears  to  be  due 
to  the  small  amount  of  iron  left  in  the  print,  and  by 
the  application  of  dilute  acid  and  chlorine  it  can  be 
entirely  removed  and  the  original  coloui  restored  in  all 
its  brilliance. 

Thus  the  platinum  image  is  not  only  able  to  resist  all 
detrimental  atmospheric  influences,  but  there  is  no  method 
known  by  which  it  can  be  attacked.  As  the  sensitive 
substance  is  put  directly  upon  the  paper,  the  paper  must 
be  of  excellent  quality,  free  from  extraneous  matter  and 
fibres  of  inferior  quality,  for  if  any  such  matteis  were 
present  they  would  affect  the  process  and  show  flaws 
in  the  print.  The  suitability  of  the  paper  is  therefore  a 
practical  guarantee  of  its  purity,  and  as  we  know  that 
linen 1  will  last  for  thousands  of  years,  as  demonstrated 
for  example  in  the  present  condition  of  the  cloths  on 
mummies,  there  seems  every  reason  to  suppose  that  if 
platinum  prints  had  been  made  in  Abraham  s  time,  or 
when  Egypt  was  at  the  height  of  its  glory,  they  might,  if 
preserved  with  reasonable  care,  have  been  available  for 

our  information  at  the  present  day. 

Platinum  printing  is  essentially  an  iron  process,  because 
it  is  the  iron  compound  that  is  changed  by  exposure  to 
the  light,  and  it  is  the  new  iron  compound  (the  ferrous 
salt)  which  at  a  second  stage  of  the  process  acts  upon 
the  platinum  salt  and  gives  a  deposit  of  the  metal.  The 
platinum  salt  may  therefore  be  omitted  from  the  coating 
on  the  paper  and  be  applied  after  the  exposure,  and  a 
practical  method  was  actually  worked  out  on  these  lines 
a  short  time  ago,  but  it  was  withdrawn  as  it  was  not 
on  the  whole  advantageous.  Instead  of  a  platinum  salt 
which  gives  an  image  in  metallic  platinum,  a  silver  salt 
may  be  used  and  the  image  obtained  in  metallic  silver, 

i  Linen  and  paper  consist  almost  entirely  of  the  same  substance,  namely, 
cellulose. 

IQO 


Other  Printing  Methods 

or  a  gold  salt  will  give  an  image  in  metallic  gold.  These 
processes  have  been  dignified  with  names,  but  the  silver 
method  is  not  used  because  other  methods  of  getting  silver 
images  are  preferable,  and  the  gold  method  is  useless 
because  gold  images  have  so  little  substance  or  body  in 
them,  and  they  are  generally  of  unsuitable  colours.  For 
such  methods  as  these  last  a  citrate  instead  of  an  oxalate 
of  iron  is  often  preferred,  but  the  principle  of  the  method 
and  the  character  of  the  change  produced  by  light  is 
exactly  the  same. 

If  paper  coated  with  the  iron  salt  only  is  exposed  under 
a  negative  and  then  plunged  into  a  solution  of  potassium 
ferricyanide,  the  ferrous  salt  that  the  light  has  produced 
will  give  a  dark  blue  insoluble  deposit  with  the  ferricyanide, 
while  the  original  salt  unacted  on  by  light  will  give  only 
a  soluble  product.  If  then  everything  is  washed  away 
that  water  will  dissolve,  a  dark  blue  image  is  obtained,  the 
colour  of  which  cannot  be  distinguished  by  the  eye  from 
Prussian  blue  and  indeed  is  quite  analogous  to  it.  The 
ferricyanide  may  be  mixed  with  the  iron  salt  and  the  paper 
coated  with  the  mixture,  then  after  exposure  the  print 
needs  only  to  be  washed  in  water  to  give  the  blue  image. 
Paper  of  this  character  is  extensively  used  for  copying 
engineers’  and  architects’  drawings,  because  as  no  costly 
substances  are  used  in  its  preparation  it  is  cheap,  and  its 
manipulation  is  very  simple.  The  fact  that  the  image  is 
blue  is  no  detriment  for  such  purposes,  but  those  who 
are  acquainted  with  the  chemistry  of  iron  compounds 
would  find  no  difficulty  in  modifying  the  colour  if  the 
blue  were  objectionable. 

The  use  of  iron  compounds  as  sensitive  substances 
gives  a  great  variety  of  possibilities  to  the  photographer, 
for  there  are  other  applications  of  them  to  which  we  have 
not  referred,  but  the  importance  of  these  is  far  surpassed, 
so  far  as  practical  work  is  concerned,  by  potassium  bichro¬ 
mate.  We  shall  see  in  this  and  the  following  chapter  in 

197 


Other  Printing  Methods 

how  many  different  ways  this  substance  is  utilised.  When 
a  gelatine  film  is  impregnated  with  it,  the  gelatine  remains 
soluble  in  hot  water,  but  if  the  impregnated  film  is  exposed 
to  light,  the  bichromate  is  decomposed  and  the  product 
of  its  decomposition  renders  the  gelatine  insoluble  in 
water.  If  therefore  a  bichromated  film  is  exposed  in  some 
parts  and  not  in  others,  it  may  be  “  developed  ”  by  means 
of  hot  water,  and  if  the  gelatine  had  previously  been  mixed 
with  a  pigment  such  as  lamp-black,  those  paits  of  the 
gelatine  film  that  had  been  made  insoluble  by  the  exposure 
would  remain  with  the  pigment,  while  the  rest,  would  be 
washed  away.  This  is  the  essence  of  “carbon’'  or  “pig¬ 
ment”  printing,  and  the  procedure  was  patented  in  1855 
by  Poitevin.  It  seems  simple  at  first  sight,  but  a  little 
consideration  will  show  that  this  crude  process  will  not 
work.  The  insolubility  obviously  begins  at  the  surface 
of  the  gelatine  and  penetrates  downwards  into  the  film 
as  the  light  continues  to  act.  If  in  the  darkest  paits  of 
the  print  the  light  has  produced  insolubility  right  through 
the  film,  in  all  the  less  dark  parts  which  represent  the 
middle  tones  and  the  high  lights,  the  insolubility  will  not 
extend  to  the  lower  surface  of  the  film,  and  the  insoluble 
gelatine  will  lie  upon  that  portion  that  remains  soluble 
beneath  it.  When  the  hot  water  is  applied,  it  dissolves 
this  soluble  part,  and  the  insoluble  part  supported  by  it 
floats  away.  In  1858  it  was  found  that  this  difficulty  could 
be  remedied  by  exposing  through  the  paper  upon  which 
the  gelatine  film  was  supported,  as  then  all  the  parts  made 
insoluble  were  actually  on  the  paper.  Good  “  carbon  ”  or 
u  pigment"  prints  were  made  in  this  way,  but,  to  say  the 
least  of  it,  it  was  an  awkward  method,  and  the  next  step 
in  advance  was  to  expose  from  the  front  as  usual,  and  to 
coat  the  print  with  collodion  before  it  was  developed. 
In  the  hot  water  the  film  of  insoluble  pigmented  gelatine 
would  float  off  from  the  paper  on  which  it  was  originally, 
but  the  collodion  held  it  together,  and  it  could  be  caught 


Other  Printing  Methods 

upon  another  sheet  of  paper  coated  with  gelatine  to  form 
the  final  print.  Thus  the  principles  of  carbon  printing 
were  well  understood  at  this  time,  but  it  was  not  until 
1864,  when  Mr.  J.  W.  Swan  introduced  a  few  modifica¬ 
tions,  that  the  process  became  suitable  for  commercial 
work  on  a  considerable  scale.  The  method  as  at  present 
practised  is  Sir  J.  W.  Swan's,  though  it  has  been  improved 
in  a  few  details. 

“  Carbon  tissue  ”  is  paper  coated  with  a  substantial  layer 
of  gelatine  which  has  been  mixed,  before  application  to 
the  paper  and  while  in  solution  in  warm  water,  with  a 
sufficiency  of  any  suitable  pigment.  The  only  restrictions 
as  to  the  character  of  the  pigment  that  can  be  used  is  that 
it  must  not  injure  the  gelatine,  and  it  must  be  unaffected 
by  either  warm  water  or  potassium  bichromate  or  alum, 
and  this  gives  such  a  wide  choice  that  it  is  possible  to 
prepare  a  tissue  of  almost  any  colour  or  shade.  The 
bichromate  that  confers  the  sensitiveness  may  be  mixed 
with  the  melted  jelly  before  the  paper  is  coated,  or  the 
coated  paper  may  be  treated  with  a  solution  of  the  bi¬ 
chromate.  The  sensitive  tissue  is  exposed  to  light  under  the 
negative  as  usual,  but,  as  the  pigment  in  the  gelatine  makes 

I  it  so  dark,  the  slight  change  of  colour  that  light  produces 
in  the  potassium  bichromate  is  not  visible.  The  progress 
of  the  action  of  the  light  is  therefore  gauged  by  an  acti- 
nometer,  in  which  a  small  surface  of  sensitive  silver  paper 
is  exposed  to  light  through  a  hole  in  a  plate  that  covers  it, 
and  the  darkening  of  the  paper  is  watched  until  it  is  equal 
to  a  painted  tint  by  the  side  of  the  hole.  The  exposure 
of  the  carbon  print  may  have  to  be  continued  while  the 
silver  paper  in  the  actinometer  darkens  to  the  comparison 
colour  several  times  according  to  the  density  of  the 
negative  and  the  print  required.  A  piece  of  paper  with 
a  smooth  coating  of  insoluble  gelatine  on  one  side  of  it,  is 
plunged  into  cold  water  with  the  exposed  carbon  tissue  ; 
after  a  minute  or  two  they  are  withdrawn  from  the  water 

199 


Other  Printing  Methods 

face  to  face  in  contact,  and  the  water  between  them  is 
forced  out  by  putting  them  on  a  flat  surface  and  stroking 
them  repeatedly  with  a  flexible  edge  of  india-rubber. 
The  two  papers  now  stuck  together  are  next  put  into 
warm  water,  which  soon  penetrates  to  the  soluble  gelatine 
and  begins  to  dissolve  it,  so  that  the  original  paper  may 
be  stripped  off,  leaving  the  bulk  of  the  pigmented  gelatine 
on  the  second  sheet,  with  what  was  the  face  of  the  gelatine 
during  the  exposure  secured  to  it.  The  warm  water 
gradually  dissolves  away  the  gelatine  that  has  been  left 
soluble  by  the  exposure  and  the  picture  gradually  appears. 
When  fully  developed,  the  print  is  put  into  alum  solution 
to  harden  the  gelatine,  washed,  and  hung  up  to  dry. 

A  carbon  print  made  as  just  described  is  called  a 
single  transfer  print,  because  the  exposed  gelatine  film 
is  transferred  once,  namely,  from  the  paper  that  carried 
it  during  the  exposure  to  the  paper  upon  which  it  was 
developed  and  finally  remains  as  the  finished  print.  The 
effect  of  this  transference  is  of  great  importance.  If 
the  reader  will  write  on  a  piece  of  paper,  and,  while  the 
writing  is  still  wet,  put  blotting-paper  upon  it,  the  writing 
will  be  partially  transferred  to  the  blotting-paper,  but 
the  transferred  writing  will  be  laterally  inverted,  a  con¬ 
dition  that  has  been  described  in  detail  at  page  87. 
The  writing  will  progress  from  right  to  left  instead  of 
from  left  to  right,  and  every  letter  will  be  as  it  were 
turned  over  sideways.  The  single  transfer  print  will 
be  laterally  inverted  in  exactly  the  same  way,  and  the 
effect  in  a  portrait,  for  example,  would  be  to  make  the 
person  appear  left-handed,  and  to  change  over  every 
detail  from  right  to  left  and  left  to  right.  Such  inversion 
is  of  course  not  to  be  permitted.  If  the  writing  on  the 
blotting-paper  is  held  up  in  front  of  a  looking-glass,  the 
reflection  will  be  seen  to  be  correct,  because  the  mirror 
produces  a  second  lateral  inversion,  as  we  have  already 
seen.  When  many  carbon  prints  are  wanted,  as  in  the 

200 


Other  Printing  Methods 


reproduction  of  pictures  for  publication  and  other  similar 
cases,  it  is  customary  to  correct  the  inversion  by  photo¬ 
graphing  the  reflection  of  the  picture  instead  of  the 
picture  itself.  The  mirror  is  put  close  to  the  lens,  because 
in  that  position  it  needs  to  be  but  little  larger  than  the 
lens  itself,  and  it  is  fixed  at  an  angle  of  45  degrees.  The 
negative  so  produced  would  give  a  laterally  inverted  piint 
by  any  direct  printing  process  like  those  we  have  pre¬ 
viously  considered,  but  the  transfer  in  the  carbon  process 
puts  the  image  into  the  correct  position.  The  two  inver¬ 
sions  are  exactly  analogous  to  turning  a  piece  of  paper 
over  twice,  when  the  second  turn  restores  it  to  its  original 
position. 

But  if  a  carbon  print  is  wanted  from  an  ordinary 
negative,  then  the  necessary  second  inversion  is  obtained 
by  a  second  transfer.  The  exposed  tissue  is  put  down 
upon  a  “temporary  support"  for  development  instead  of 
upon  the  paper  it  is  to  remain  on,  and  retransferred  from 
this,  after  development,  to  the  final  support.  The  surface 
of  the  temporary  support  is  so  prepared  by  thoroughly 
cleaning  it  and  then  giving  it  an  imperceptible  coating 
of  wax  which  is  almost  entirely  polished  off,  that  it  will 
hold  fcthe  picture  securely,  but  not  quite  so  tenaciously 
as  the  final  support.  This  is  coated  with  a  solution  of 
slightly  hardened  gelatine,  and  when  it  and  the  developed 
picture  are  thoroughly  soaked  in  water  they  are  brought 
together  under  water,  withdrawn,  and  as  much  as  possible 
of  the  water  between  them  is  pushed  out  by  a  sweeping 
movement  of  an  india-rubber-edged  scraper  (called  a 
squeegee).  When  quite  dry,  the  two  supports  are  sepa¬ 
rated  and  the  print  adheres  firmly  to  the  final  support. 
The  temporary  support  generally  used  is  flexible,  some¬ 
what  like  thick  paper  with  a  hard,  smooth  surface,  and 
the  final  support  may  be  almost  any  surface,  as  paper, 
celluloid,  ivory,  opal  glass,  porcelain,  plain  glass  for  trans¬ 
parencies,  &c.  As  the  picture  is  thoroughly  washed 

201 


Other  Printing  Methods 

during  development  on  its  temporary  support,  there  is 
no  soluble  matter  left  in  it  to  contaminate  the  surface 
to  which  it  is  transferred. 

There  is  a  peculiar  circumstance  that  has  to  be  borne 
in  mind  when  dealing  with  gelatine  that  has  been  made 
sensitive  to  light  by  means  of  a  bichromate.  If  we  may 
compare  inanimate  things  with  animate,  we  may  say 
that  of  those  substances  that  are  acted  upon  by  light  there 
are  some  that  change  sulkily  and  unwillingly,  and  as  soon 
as  the  force  that  obliges  them  to  alter  is  withdrawn  they 
revert  to  their  original  condition.  There  are  others  that 
welcome  change,  and  if  only  they  are  started  will  continue 
to  alter  their  condition  even  after  the  force  is  withdrawn. 
And  there  are  others  that  take  things  as  they  come  and 
care  nothing  either  one  way  or  the  other.  The  *gelatino- 
bromide  plate  belongs  to  the  last  section,  for  it  will  re¬ 
main  as  it  is  for  years,  and  if  exposed  for  the  production 
of  a  negative,  it  will  remain  in  its  changed  condition  for 
years,  ready  for  development.  Gelatine  with  bichromate 
belongs  to  the  second  section.  The  change  only  needs 
to  be  started  by  light  and  it  will  continue  when  the  light 
is  withdrawn,  though  obviously  not  so  quickly  as  if  the 
light  were  still  at  work  upon  it.  This  continuing  action 
is  of  very  practical  importance  and  must  be  allowed  for 
if  development  does  not  take  place  within  an  hour  or 
so  of  exposure.  If  a  print  has  had  only  the  tenth  of 
the  necessary  exposure  and  it  is  put  away  in  the  dark 
for  a  day  or  perhaps  two  days,  it  will  be  found  to  be 
as  if  fully  exposed.  This  circumstance  has  been  utilised 
when  the  light  is  poor  or  many  prints  are  wanted  from 
the  same  negative,  but  as  the  rate  at  which  the  change 
continues  depends  upon  the  hygrometric  condition  of  the 
gelatine  and  perhaps  upon  other  circumstances  which  are 
practically  uncontrollable,  the  result  is  very  uncertain,  and 
at  the  present  time  is  probably  never  taken  advantage  of. 
If  the  partly  exposed  print  is  kept  really  dry  by  chemical 

202 


Other  Printing  Methods 

means,  by  shutting  it  up  with  a  powerful  absorbent  of 
moisture,  then  the  change  is  arrested. 

The  permanency  of  a  carbon  print  obviously  depends 
upon  the  character  of  the  pigment  used.  At  first,  when 
the  peculiar  colours  of  silver  prints  were  imitated,  suffi¬ 
cient  care  was  not  exercised  in  this  direction,  the  pigments 
were  mixtures  containing  fugitive  constituents,  and  of 
course  the  colour  of  the  prints  altered  in  the  course  of 
time.  A  more  judicious  choice  is  made  at  the  present 
time,  but  this  possibility  of  the  presence  of  an  unstable 
substance  must  always  remain,  and  the  photographer  has 
to  trust  to  the  maker  of  the  tissue.  If  carbon  only,  such 
as  Indian  ink  or  lamp-black,  is  used,  then  the  pigment  is 
thoroughly  satisfactory,  and  so  far  as  permanency  is  con- 

Icerned  we  have  to  consider  only  the  gelatine  film  that 
carries  it  and  the  paper  that  supports  the  film.  The  paper 
need  not  be  of  that  perfect  quality  that  is  necessary  in 
platinum  printing,  because  the  image  is  not  produced  or 
mounted  in  immediate  contact  with  it,  and  the  very  fact 
that  there  is  a  film  to  carry  the  image  is  an  added  com¬ 
plication.  A  carbon  print  has  therefore  more  points  of 
possible  attack  than  a  platinum  print,  and  although 
platinum  and  carbon  may  well  be  associated  together  as 
the  two  processes  that  yield  permanent  results,  platinum 
must  always  be  the  more  desirable  when  the  distant  future 
;  is  considered. 

The  carbon  process  is  one  that  lends  itself  to  many 
modifications,  some  of  which  are  curious  and  some  useful. 

;  As  an  example  of  the  former  the  development  by  digestion 
instead  of  by  hot  water  may  be  noted.  By  putting  the 
print  into  water  containing  about  one  per  cent,  of  pepsin 
and  one  and  a  half  per  cent,  of  hydrochloric  acid  at  the 
|  temperature  of  a  moderate  summer  s  day,  the  gelatine 
that  remains  soluble  in  hot  water  will  be  attacked  and 

dissolved  away  in  about  three  hours. 

The  ozotype  process  was  introduced  by  Mr.  Thomas 

203 


Other  Printing  Methods 

Manly  in  1899,  and  has  the  advantage  that  there  is  no 
transfer  of  the  image  produced  by  exposuie,  and  there¬ 
fore  no  question  of  lateral  inversion,  and  as  the  effect  of 
the  exposure  to  light  is  visible,  there  is  no  need  of  an 
actinometer.  The  pigment  in  its  film  of  gelatine  is  not 
made  sensitive  at  all  as  in  the  ordinary  carbon  process. 
The  paper  upon  which  the  print  is  to  be  is  suitably  sized 
and  rendered  sensitive  with  a  solution  of  potassium 
bichromate,  mixed,  however,  with  a  manganese  salt.  It  is 
exposed  to  light  under  the  negative  until  the  light  brown 
image  is  of  suitable  density,  and  then  washed  until  all 
the  unattacked  bichromate  is  removed.  The  print  is 
now  not  sensitive  to  light  and  may  remain  for  any  con¬ 
venient  time,  up  to  about  two  months,  before  it  is  pro¬ 
ceeded  with.  To  complete  the  print,  the  exposed  sheet 
and  a  sheet  coated  with  pigmented  gelatine  are  immersed 
in  water  in  which  have  been  dissolved  a  little  acid,  a 
little  alum,  and  some  ferrous  sulphate,  brought  out  of 
the  solution  face  to  face,  and  the  excess  of  the  liquid 
is  driven  out  from  between  them  with  a  squeegee.  In 
half  an  hour  or  so  the  print  is  put  into  hot  water,  the 
paper  backing  of  the  gelatine  is  stripped  off  and  de¬ 
velopment  completed  as  in  the  ordinary  process.  During 
the  waiting  before  development,  the  acid  dissolves  the 
chromium  salt  that  has  been  changed  by  light  and  so 
enables  it  to  slowly  penetrate  into  the  overlying  film  of 
pigmented  gelatine  and  render  insoluble  those  parts  that 
it  comes  into  contact  with.  The  other  substances  pre¬ 
sent  assist  or  serve  to  regulate  the  changes  that  take 
place. 

A  few  years  after  Mr.  Manly  introduced  the  ozotype 
process,  he  devised  the  ozobrome  process,  by  means  of 
which  an  ordinary  bromide  print  may  be  changed  into 
a  carbon  print.  This  method  offers  the  considerable 
advantage  that  the  exposuie  necessary  is  only  a  few 
seconds,  or  it  may  be  a  minute  or  so,  to  artificial  light, 

204 


Other  Printing  Methods 

instead  of  the  prolonged  exposure  to  daylight  necessary 
when  the  bichromated  gelatine  itself  is  the  compound 
acted  upon  by  the  light.  Thus  it  is  possible  from  a 
small  negative  to  make  an  enlarged  carbon  print  with¬ 
out  the  need  for  an  enlarged  negative,  which  is  neces¬ 
sary  for  the  preparation  of  a  carbon  enlargement  in 
the  ordinary  way.  If  the  bromide  print  is  already  pre¬ 
pared,  no  exposure  to  light  is  necessary.  A  sheet  of 
paper  that  carries  a  film  of  gelatine  is  soaked  in  a  solution 
containing  a  bichromate,  a  ferricyanide,  and  a  bromide, 
and  then  pressed  into  close  contact  with  the  bromide 
print.  After  a  few  minutes  the  print  may  be  developed 
in  hot  water  as  if  it  were  an  ordinary  carbon  print. 
The  silver  of  the  bromide  print  produces  an  effect  in 
the  bichromate  similar  to  the  effect  of  light,  and  so  it 
indirectly  renders  the  gelatine  insoluble  and  fixes  upon 
itself  a  due  proportion  of  the  pigmented  gelatine  that 
resists  the  action  of  the  hot  water.  Instead  of  simple  de¬ 
velopment,  the  two  papers  may  be  peeled  apart ;  the  one 
that  bears  the  pigmented  gelatine  may  be  treated  exactly 
as  an  ordinary  exposed  carbon  print,  that  is,  mounted 
and  developed,  while  the  bromide  print  which  has  its 
silver  now  changed  into  a  compound  of  silver  may  be 
brought  back  to  its  original  condition  by  any  ordinary 
developer,  and  will  then  furnish  other  carbon  prints  by 
repeating  the  process,  all  without  any  need  for  exposure 
to  light. 

About  four  years  ago  the  Rotary  Photographic  Com¬ 
pany  put  upon  the  market  a  bromide  paper  that  had 
the  pigment  in  the  film  with  the  silver  bromide.  The 
I  paper  was  exposed  and  developed  as  for  a  bromide  print, 
then  put  into  a  bichromate  solution  for  a  suitable  time 
and  washed.  The  metallic  silver  of  the  developed  image 
acted  upon  the  bichromate,  and  the  print  was  mounted 
j  and  developed  like  an  ordinary  carbon  print,  thus  getting 
a  carbon  enlargement  with  the  same  facilities  concerning 

205 


Other  Printing  Methods 

the  exposure  as  if  only  a  bromide  print  were  being 
prepared,  and  by  the  use  of  only  a  single  sheet  of 

coated  paper. 

It  was  pointed  out  above  that,  as  the  light  when  it 
acts  on  a  film  of  gelatine  made  sensitive  by  bichromate 
of  potassium  produces  insolubility  from  the  outer  surface 
of  the  film  downwards,  it  is  necessary  to  support  the 
outer  surface  of  the  film  during  development  to  prevent 
it  from  being  washed  away  by  the  solution  of  the  soluble 
gelatine  beneath  it.  But  these  conditions  may  be  modified 
if  the  film  is  kept  thin  and  allowed  to  enter  into  the 
paper  that  supports  it  to  a  slight  extent.  About  twenty 
years  ago  M.  Artigue  introduced  a  kind  of  carbon  tissue, 
«  papier-velours,”  which  after  exposure  was  developed  as 
it  was,  without  transfer,  by  pouring  over  it  a  kind  of 
soup  made  of  fine  sawdust  and  water,  the  slight  friction 
carrying  away  the  parts  not  made  sufficiently  insoluble 
by  the  "exposure  to  light  to  withstand  the  action.  The 
method  by  which  this  paper  was  prepared  has  not  been 
published,  but  it  led  to  a  revival  of  the  gum-bichromate 
process  which  Pouncy  had  worked  at  long  before.  The 
paper  is  coated  in  this  case  with  the  pigment  mixed  with 
a  gum  mucilage  rendered  sensitive  with  the  bichromate, 
exposed  under  the  negative,  and  developed  by  passing 
water  over  it,  and  assisting  the  removal  of  the  still 
soluble  gum  with  the  pigment  it  contains  by  the  appli¬ 
cation  of  a  soft  brush.  Gum,  unlike  gelatine,  does  not 
require  warm  water  to  dissolve  it,  and  as  the  insolubility 
produced  by  exposure  is  relative  rather  than  absolute, 
more  or  less  can  be  removed  in  the  various  parts  of  the 
picture  at  the  pleasure  of  the  worker.  Such  selective 
development  partakes  of  the  nature  of  painting  by  hand 
but  in  the  reverse  direction,  because  the  effect  of  the 
brush  is  to  remove  the  pigment  and  not  to  add  it.  1  his 
possibility  of  control  is  much  valued  by  those  who  are 
interested  in  photography  chielly  or  wholly  as  a  method 

206 


Distortion  at  the  edges  of  the  Plate  due  to  its  Flatness 

The  rays  that  form  the  image  fall  obliquely  on  the  edges  of  the  P1^’  ^  of^sXre 
is  thus  drawn  out  in  a  radial  direction.  The  upper  figure  is  the  photograph  of  a  sphere 

to  his  height,  and  this  effect  increases  towards  the  edges  of  the  plate. 


Other  Printing  Methods  . 

of  pictorial  expression,  because  it  enables  them  to  sup¬ 
plement  photography  with  such  handwork  as  their  skill 
and  artistic  feeling  may  indicate  as  desirable.  Possibly 
also  it  is  valued  by  some  as  a  means  of  correcting  the 
errors  that  they  have  committed  in  the  photographic 
process. 

In  the  methods  considered  so  far  the  pigment  is  put 
into  the  gelatine  or  gum  before  the  film  is  made,  the  film 
is  applied  as  a  whole,  and  what  is  not  wanted  is  dis¬ 
solved  away.  There  are  also  methods  of  getting  pigment 
prints  in  which  the  pigment  is  applied  to  the  film  after 
the  exposure  to  light.  If  a  film  of  gelatine  made  sensi¬ 
tive  to  light  by  means  of  potassium  bichromate  is  exposed 
to  light  under  a  negative  and  then  put  into  water,  those 
parts  where  the  light  has  acted  will  absorb  less  water 
than  the  other  parts,  and  if  the  surface  water  is  wiped 
away  in  proportion  as  the  parts  absorb  less  water  so 
will  they  the  more  rapidly  allow  of  the  deposition  of 
greasy  matter  upon  them.  The  greasy  matter  may 
contain  pigment,  it  may  in  fact  be  an  oil  paint  or 
printers’  ink.  This  fact  has  been  known  for  some  fifty 
years  or  so,  but  it  is  only  quite  recently,  in  1904,  that 
Mr.  G.  E.  H.  Rawlins  proposed  to  utilise  the  process 
for  the  preparation  of  photographic  prints.  Paper  coated 
with  gelatine  is  put  into  a  solution  of  potassium  bichro¬ 
mate  to  sensitise  it,  dried,  exposed  under  the  negative, 
washed  in  several  changes  of  water  to  remove  the  un¬ 
changed  bichromate,  well  soaked  in  water  and  wiped  dry 
on  its  surface.  It  is  now  ready  to  receive  the  ink.  This 
may  be  dabbed  on  with  a  pad,  or  rolled  on,  or  brushed 
on.  If  dabbed  on  all  over  the  print  and  an  ordinary 
printers’  roller  is  passed  lightly  over  it  a  few  times,  the 
paint  will  gradually  leave  the  unexposed  parts  and  ac¬ 
cumulate  on  the  shadows.  By  letting  a  moderately  stiff 
brush  fall  upon  the  print  from  a  height  of  about  an  inch 
and  wiping  the  brush  as  required,  paint  may  be  removed. 

207 


Other  Printing  Methods 

By  such  methods  as  these  the  pigment  may  be  applied 
according  to  the  taste  of  the  worker. 

In  1908,  Mr.  C.  Welborne  Piper  devised  a  practical 
method  of  applying  this  process  to  bromide  prints,  after 
the  manner  in  which  Mr.  Manly  had  adapted  his  ozotype 
process.  The  bromide  print  is  bleached  in  a  solution 
similar  to  Mr.  Manly’s  but  with  a  little  alum  and  citric  acid 
added  to  it,  soaked  for  a  short  time  in  dilute  sulphuric  acid 
and  then  the  silver  salt  is  dissolved  out  in  a  fixing  bath. 
The  metallic  silver  of  the  original  image  has  by  the  aid 
of  the  first  solution  used,  which  contains  the  bichromate, 
acted  upon  the  gelatine  and  bichromate  very  much  as 
light  would  have  done,  so  that  wherever  the  silver  was 
and  in  proportion  to  its  quantity,  the  gelatine  is  less  ready 
to  absorb  water  and  more  ready  to  receive  a  greasy  ink. 
The  application  of  the  pigment  is  exactly  as  before,  and 
the  advantage  is,  as  in  the  case  of  ozobrome,  that  the  very 
small  exposure  to  light  necessary  for  the  production  of  a 
bromide  print  suffices  instead  of  the  protracted  exposure 
necessary  when  the  light  acts  directly  upon  the  bichro¬ 
mate. 

These  processes  may  be  modified  in  detail  but  the 
principle  remains  the  same.  There  are,  however,  two  ! 
other  distinct  properties  of  potassium  bichromate  of  which  J 
good  use  has  been  made  in  the  past.  Dextrine  as  usually 
obtained  is  a  sugary,  sticky  substance,  and  if  this  is  mixed  j 
with  a  bichromate  it  will  form  a  sticky  film  upon  any 
surface  to  which  it  is  applied,  as  sticky  as  if  the  bichro¬ 
mate  were  not  present.  Exposure  to  light  changes  the 
bichromate  as  usual,  and  the  products  of  its  decom¬ 
position  diminish  the  stickiness  of  the  film  in  proportion  j 
to  their  quantity.  By  “dusting  on,”  or  moving  by  means | 
of  a  soft  brush,  a  finely  ground  pigment,  such  as  black 
lead,  over  the  surface  of  such  a  film,  the  powder  will 


Other  Printing  Methods 

adhere  in  proportion  as  the  film  remains  sticky,  and  in 
this  way  an  excellent  image  may  be  produced. 

If  paper  or  other  material  is  coated  with  or  soaked 
in  a  bichromate  solution  and  exposed  after  drying  it,  the 
bichromate  will  be  decomposed  in  proportion  to  the 
amount  of  light  action,  as  in  all  the  methods  already 
described.  Instead  of  utilising  the  products  of  decom¬ 
position  of  the  bichromate  to  render  such  substances  as 
gelatine,  gum,  or  dextrine  less  soluble  or  less  able  to 
absorb  water,  it  is  possible  to  utilise  the  unchanged 
bichromate  for  the  production  of  an  image.  The  bichro¬ 
mate  is  a  powerful  oxidiser,  and  if  allowed  to  act  upon 
aniline  will  oxidise  it  into  a  mixture  of  dark  substances, 
chiefly  tarry  matter  and  aniline  purple.  So  by  allowing 
the  vapour  of  aniline  to  come  into  contact  with  the 
exposed  bichromated  sheet,  the  parts  where  the  bichro¬ 
mate  remains  are  blackened.  It  will  be  noticed  that  in 
this  as  in  the  “  dusting  on "  process,  the  dark  parts  of 
the  prints  are  those  where  the  light  has  not  acted,  so 
that  the  print  is  a  reproduction  of  the  plate  through  which 
the  exposure  was  made,  instead  of  having  its  lights  and 
darks  reversed  as  is  more  usual.  For  this  reason,  the 
“  dusting  on  "  process  has  been  used  for  the  reproduction 
of  negatives,  and  the  aniline  process  for  the  copying  of 
engineers’  drawings. 

These  latter  processes,  and  others  that  might  be 
mentioned,  are  probably  never  used  at  the  present  time. 
This  neglect  of  methods  of  work  that  have  done  excellent 
service  is  due  to  a  kind  of  survival  of  the  fittest.  The 
misfortune  is  that  the  “  fittest  ”  is  not  always  the  really 
best,  but  merely  the  one  that  happens  to  best  “  fit  ”  the 
existing  conditions,  one  of  which  may  be  a  general  want 
of  skill  and  knowledge.  Processes  that  have  passed  into  dis¬ 
use  must  therefore  not  always  be  condemned  as  inferior 

209  O 


Other  Printing  Methods 

to  the  others  that  have  taken  their  places.  The  greater 
number  of  those  who  practise  photography  now  want  as 
much  as  possible  done  for  them,  and  the  little  that  they 
have  to  do  to  be  made  as  simple  and  as  mechanical  as 
possible.  This  fact  will  account  for  many  circumstances 
in  connection  with  photographic  work. 


210 


CHAPTER  XIII 


PHOTO-MECHANICAL  PRINTING 

The  problem  of  multiplying  copies  of  photographs  was 
present  in  the  minds  of  many  of  the  pioneers  of  the  art. 
The  preparation  of  a  negative  from  which  prints  may  be 
made  by  the  photographic  processes  described  in  the 
previous  chapters  was  a  step  in  this  direction.  If  a  large 
number  of  prints  is  needed,  as  for  publication  purposes,  it 
is  usual  to  make  duplicate  negatives  so  that  several  prints 
may  be  simultaneously  produced,  but  in  any  case  the 
operation  is  tedious  and  the  results  comparatively  costly, 
especially  if  a  “printing-out”  method  is  employed.  It 
was  realised  nearly  a  hundred  years  ago  that  it  was 
desirable  to  be  able  to  prepare  a  plate  by  photographic 
means  that  would  enable  impressions  to  be  obtained  in  an 
ordinary  printing  press,  so  that  the  accuracy  and  rapidity 
of  photography  might  be  combined  with  the  facility  of 
common  printing  methods.  The  elder  Niepce  did  some¬ 
thing,  Fox  Talbot  did  a  good  deal  more  of  work  in  this 
direction,  and  in  the  early  fifties  there  was  realised  a 
measure  of  success  that  was  distinctly  encouraging  and 
that  soon  brought  innumerable  workers  into  the  field.  In 
one  direction  it  was  sought  to  photograph  directly  on  to 
the  block  for  the  wood  engraver  instead  of  drawing  upon 
it  by  hand,  but  this  kind  of  work  is  now  very  nearly 
obsolete,  for  photography  has  superseded  not  only  the 
draughtsman  but  the  wood  engraver  too,  except  to  an 
exceedingly  small  extent.  Steel  and  copper  plate  engrav¬ 
ing  has  followed  wood  cutting,  and  one  may  say  in  general 

21 1 


Photo-mechanical  Printing 

language  that  all  illustrations  for  book  or  similar  work 
are&now  produced  photographically.  We  do  not  propose 
to  trace  the  history  of  the  development  of  modern 
methods  of  illustration,  for  this  is  a  very  extensive  subject, 
but  rather  to  give  some  idea  of  the  principles  upon  which 

the  chief  of  them  are  based. 

If  a  print  is  made  from  a  negative  by  any  photographic 
process,  the  result  is  generally  called  a  photographic  print 
or  simply  a  photograph  ;  but  if  from  a  photograph  or 
negative  a  plate  or  a  block  is  prepared  and  impressions  are 
taken  from  this,  the  result  is  called  a  photo-mechanical 
print,  because  although  it  has  a  photographic  origin,  the 
print  itself  is  prepared  by  mechanical  means.  We  have 
seen  that  a  carbon  print  is  prepared  by  treating  a  pig¬ 
mented  film  of  gelatine  with  potassium  bichromate,  ex¬ 
posing  to  light  under  a  negative,  and  then  dissolving  away 
with  hot  water  those  parts  of  the  film  that  have  not  been 
acted  upon  by  the  light  and  therefore  remain  soluble. 
The  darkness  of  the  image  depends  entirely  upon  the 
thickness  of  the  film  that  has  been  affected  by  light  and 
so  made  insoluble.  The  dark  shadows  are  represented 
by  a  comparatively  thick  layer  of  the  pigmented  gelatine, 
the  medium  tones  by  a  thinner  layer,  and  any  parts  of 
the  image  that  are  white  have  no  deposit  at  all  upon  them, 
for  there  it  has  all  been  dissolved  away.  The  image  in 
fact  is  in  relief,  and  the  height  of  the  relief  is  proportional 
to  its  darkness.  This  is  a  photographic  print;  but  if  an 
exactly  similar  relief  is  made  by  pouring  melted  pigmented 
gelatine  into  a  mould  and  allowing  it  to  set  and  dry,  the 
print  is  a  mechanical  print,  and  if  the  mould  is  obtained 
by  photographic  means,  it  is  a  photo-mechanical  print. 
The  “casting"  method  of  getting  prints  is  called  Wood- 
burytype,  after  the  name  of  the  inventor.  A  gelatine 
relief  is  prepared  in  very  much  the  same  way  as  a  carbon 
print  is  made,  but  in  higher  relief,  and  this  when  dry  is 
placed  with  a  sheet  of  lead  in  a  hydraulic  press,  and  the 

212 


Photo-mechanical  Printing 


1 


two  are  forced  together  until  the  relief  is  driven  into  the 
lead.  Gelatine  when  dry  is  very  tough  and  hard  and  gives 
a  clean  impression.  The  lead  plate  is  printed  from  by 
pouring  a  pool  of  melted  pigmented  gelatine  on  it,  putting 
the  sheet  of  paper  on  top,  and  subjecting  them  to  pressure 
in  an  ordinary  printing  press.  The  gelatine  sets  and  the 
paper  with  the  print  upon  it  can  be  removed  from  the 
mould  and  hung  up  to  dry.  It  is  clear  why  the  relief  used 
in  making  the  mould  must  be  “  higher  ”  or  thicker  than 
an  ordinary  print,  for  it  has  when  dry  to  produce  a  mould 
that  will  hold  enough  gelatine  jelly  to  give  the  print,  and 
the  jelly  shrinks  very  considerably  as  it  dries. 

In  the  “  Stannotype  "  process  the  need  for  the  hydraulic 
press  is  dispensed  with.  The  gelatine  relief  is  made  as 
before  and  the  mould  is  built  up  on  it  by  passing  it  with 
a  sheet  of  tinfoil  between  india-rubber  rollers,  so  that  the 
foil  is  pressed  all  over  into  close  contact  with  its  surface, 
and  this  tinfoil  is  then  strengthened  by  the  deposition 
of  a  metal  upon  it  and  backing  it  up  with  a  suitable 
resinous  body.  Or  the  gelatine  relief  may  itself  be  used 
as  the  mould  by  coating  it  with  india-rubber  cement  and 
covering  it  with  tinfoil  pressed  thoroughly  down  upon  its 
surface. 

There  is  one  important  feature  of  Woodburytype 
prints,  namely  that  they  are  exactly  analogous  when 
finished  to  photographic  prints  made  by  the  carbon  pro¬ 
cess.  They  give  a  perfect  gradation  of  unbroken  tint,  thus 
resembling  photographic  prints  and  differing  from  photo¬ 
mechanical  prints  produced  by  any  other  process.  The 
“ink"  is  not  what  is  usually  understood  by  that  term,  but 
gelatine  mixed  with  the  required  pigment.  In  all  othei 
mechanical  processes  printers*  ink  is  used,  that  is  a  mixture 
that  has  the  general  characteristics  of  an  oil  paint. 

The  “oil  printing"  methods  described  in  the  last 
chapter  show  that  when  a  gelatine  film  is  sensitised  by 
means  of  potassium  bichromate  and  exposed  to  light,  that 

213 


Photo-mechanical  Printing 

in  proportion  as  the  light  acts  the  gelatine  loses  its  power 
to  absorb  water  and  gains  in  its  power  to  retain  a  greasy 
ink  (or  oil  paint).  If  now  the  gelatine  film  is  imagined 
as  on  a  thick  glass  plate  instead  of  paper,  and  the  appli¬ 
cation  of  the  ink  to  be  done  with  a  roller,  then  by  putting 
a  sheet  of  paper  upon  the  inked-up  plate  and  pressing  them 
together,  the  paper  will  take  the  ink.  The  inking-up  and 
the  taking  of  impressions  on  paper  may  be  repeated,  and 
we  have  a  photo-mechanical  method  of  printing.  This 
is  in  essence  the  process  known  as  “collotype,"  a  most 
excellent  method,  though  not  so  much  used  in  this  country 
now  perhaps  because  the  varying  condition  of  the  atmos¬ 
phere  with  regard  to  its  moisture  affects  any  gelatine 
surface  and  introduces  uncertainties  that  appear  to  be 
difficult  to  cope  with.  It  will  not  be  supposed  that  the 
preparation  of  a  collotype  plate  is  so  simple  a  matter  as 
just  described.  In  the  earliest  attempts,  about  1865, 
although  the  film  was  hardened,  only  some  four  or  five 
dozen  prints  could  be  obtained  from  the  plate  before  the 
surface  deteriorated  to  a  marked  extent.  But  by  degrees 
the  adhesion  of  the  film  to  the  glass  plate  was  improved 
and  the  process  was  perfected  in  other  directions.  It 
might  be  at  first  supposed  that  in  this  case  there  was  a 
continuous  film  of  ink  thicker  in  the  darker  parts  and 
thinner  in  the  lighter.  But  printers’  ink  is  practically 
opaque,  and  although  a  little  difference  in  intensity  in 
the  impression  can  be  obtained  by  a  slight  variation  of 
pressure,  it  is  not  possible  in  this  way  to  get  more  than 
an  exceedingly  restricted  range  of  gradation.  And  besides 
this,  the  ink  would  not  “  hold  ”  well  on  a  perfectly  smooth 
gelatine  surface.  The  fact  is  that  the  surface  of  the 
gelatine  in  a  collotype  plate  is  not  continuous,  but  is 
broken  up  into  innumerable  little  cracks,  reticulated  as 
it  is  called,  and  a  part  of  the  skill  in  the  preparation  of 
a  collotype  plate  is  shown  in  getting  the  reticulation  of  a 
suitable  fineness.  A  print  from  an  example  of  very  coarse 

214 


Photo-mechanical  Printing 

reticulation  is  given  in  the  illustration  facing  page  270* 
and  similar  markings  to  these  may  be  seen  in  any  collo¬ 
type  print  if  it  is  examined  with  a  magnifying  glass.  The 
coarser  the  reticulation  is  the  more  easy  will  the  printing 
be,  and  a  great  deal  too  depends  upon  the  character  of 
the  rollers  and  the  manner  of  using  them,  for  while  a  slow 
and  heavy  application  will  deposit  ink,  a  quicker  and 
lighter  movement  will  remove  it. 

In  any  case  a  gelatine  film  requires  to  be  treated 
with  more  care  than  a  stone  or  zinc  surface,  and  the 
greater  care  means  slower  work.  In  lithography,  which 
is  printing  from  a  stone  surface,  the  design  or  picture  is 
obtained  upon  the  stone  in  a  greasy  or  waxy  medium, 
and  as  the  stone  is  slightly  porous  the  material  enters 
to  a  small  extent  into  its  surface  and  so  becomes  firmly 
fixed.  By  sponging  the  stone  over  with  a  solution  of 
gum,  this  is  slightly  absorbed  where  the  stone  is  bare 
and  rejected  by  the  greasy  image.  The  conditions  are 
now  generally  similar  to  those  of  a  collotype  plate  \  by 
passing  an  inked  roller  over  it  the  image  will  “  take  ’  the 
ink  while  the  moistened  stone  itself  rejects  it,  and  a 
sheet  of  paper  pressed  into  contact  will  receive  an  im¬ 
pression.  A  finely  ground  zinc  or  aluminium  sheet  may 
be  used  instead  of  stone  and  the  process  is  then  called 
zincography  or  the  equivalent.  The  drawing  to  be  re¬ 
produced  may  be  made  directly  on  the  stone,  but  more 
often  it  is  made  on  a  prepared  paper  and  ‘‘transferred 
to  the  stone  or  the  zinc.  So  far  these  processes  have 
no  connection  with  photography,  but  it  is  possible  to 
produce  the  greasy  image  by  photographic  means,  and 
then  we  have  photo-lithography  or  photo-zincography. 
The  image  may  be  produced  directly  on  the  stone  or 
metal  by  coating  it  with  a  gelatinous  material  made 
sensitive  with  a  bichromate,  exposing  to  light  under  a 
negative  and  applying  a  thin  coating  of  ink  all  ovei  the 
surface.  By  washing  with  water  and  gentle  friction,  the 

215 


Photo-mechanical  Printing 

soluble  parts  of  the  film  with  the  ink  on  them  are  re¬ 
moved,  and  the  image  can  be  got  into  condition  for 
printing  from.  But  the  more  usual  method  is  to  prepare 
a  “  transfer  ”  on  paper  coated  with  gelatine  or  gelatine  and 
albumen  by  sensitising  it  with  a  bichromate,  exposing, 
and  then  applying  ink  to  it  as  already  described.  The 
transfer  is  then  laid  upon  the  stone  or  metal  sheet, 
pressed  into  contact  with  it,  and  the  paper  stripped  off. 
There  are  very  many  variations  possible  at  almost  every 
stage  of  the  process,  and  if  the  subject  is  other  than  a 
simple  line  subject,  the  method  must  be  such  as  will 
cause  the  surface  of  the  gelatine  to  reticulate  in  order 
that  the  gradation  may  be  fairly  represented.  Photo¬ 
lithography  and  photo -zincography  are  largely  used  in 
the  printing  of  maps,  and  one  advantage  of  the  latter 
method  over  the  former  is  that  the  metal  plates  are 
lighter  and  much  less  bulky  than  the  stones. 

In  collotype,  photo-lithography,  and  similar  processes 
the  printing  is  done  from  a  practically  flat  surface,  and 
the  distribution  of  the  ink  put  upon  it  is  determined  by 
the  character  of  the  surface.  It  is  true  that  there  is 
sometimes  a  very  slight  relief,  and  this  may  be  of  a  little 
assistance,  but  it  does  not  constitute  an  essential  feature 
in  the  process.  There  are  two  other  main  divisions  of 
mechanical  printing  methods,  and  both  of  them  aie 
serviceable  as  photo-mechanical  processes.  In  the  first 
case  those  parts  that  are  required  to  hold  the  ink  are 
cut  out  or  sunk  below  the  surface,  and  in  inking  the 
plate  these  depressions  are  filled  with  ink  while  the 
surface  of  the  plate  is  wiped  clean  before  the  paper  is 
put  upon  it  to  make  the  impression  ;  and  in  the  other 
the  design  stands  up  higher  than  the  remainder  of  the 
plate,  which  is  cut  away  or  otherwise  lowered,  and  the 
ink  is  put  on  by  a  roller  that  touches  only  the  project¬ 
ing  portions.  The  first  is  the  character  of  a  plate  from 
which  an  engraving  or  etching  is  printed,  while  the 


Photo-mechanical  Printing 

second  is  sometimes  referred  to  as  a  typographic  method, 
because  the  types  used  in  the  printing  of  ordinary  books 
and  such  documents  have  the  letters  and  designs  upon 
them  standing  up  in  relief.  It  might  be  thought  that 
Woodburytype  printing  came  in  the  first  category,  as 
there  the  picture  is  sunk  and  in  printing  it  is  filled  with 
the  ink.  But  Woodburytype  is  a  process  apart,  the  ink 
is  not  printers'  ink  but  a  melted  gelatine  jelly  with  which 
the  pigment  is  mixed,  and  the  mould  is  not  “  inked  in 
the  sense  in  which  a  printer  inks  a  plate;  the  pigmented 
gelatine  is  simply  poured  upon  it,  so  that  when  the 
paper  is  pressed  on  the  top  the  jelly  as  it  cools  may  set 
in  the  form  determined  by  the  mould.  Printers  ink 
cannot  be  used  in  this  way  at  all ;  it  does  not  set  like 
the  melted  jelly,  but  merely  gets  hard  by  exposure  to 
the  air  as  oil  paint  does.  A  thick  layer  of  it  would 
spread  and  run  after  the  paper  was  removed  from  the 
mould,  and  even  if  this  were  prevented  it  might  require 
weeks  or  even  months  to  harden.  On  the  other  hand, 
if  it  were  attempted  to  ink  up  a  Woodburytype  mould 
as  an  engraved  plate  is  inked,  the  process  would  fail, 
because  in  wiping  the  ink  off  the  surface  the  cloth 
would  go  into  the  hollow  parts  and  remove  the  ink 
there  also.  In  fact  there  is  no  true  surface  in  this  case 
except  round  the  edge  and  in  the  few  small  parts  that 

are  to  be  white  in  the  print. 

It  is  this  very  matter  of  surface  that  constitutes  the 
radical  difference  between  the  Woodburytype  mould  and 
an  etched  or  engraved  plate  that  can  be  printed  from  in 
printers’  ink.  The  general  surface  of  the  plate  must  be 
preserved,  and  the  lines  or  depressions  that  are  to  receive 
and  hold  the  ink  must  always  be  narrow,  and  if  a  more 
extensive  part  of  the  plate  is  required  to  hold  ink,  the  lines 
or  depressions  may  be  multiplied,  but  they  must  not  on 
any  account  run  into  each  other  so  as  to  make  a  depres¬ 
sion  of  large  area.  In  Woodburytype  a  thin  layei  of  the 

217 


Photo-mechanical  Printing 

jelly  gives  a  light  tint,  but  a  thin  layer  of  printers’  ink  will 
give  almost  its  full  colour,  and  to  get  a  light  tint  with  it,  the 
ink  must  be  alternated  with  blank  spaces  of  baie  paper. 
However  closely  a  Woodburytype  print  is  examined  it 
will  be  found  that  the  tones  are  continuous  like  the  tones 
in  a  painting  or  an  ordinary  photographic  print,  but  in 
a  print  from  an  etched  or  engraved  plate  the  tones 
are  all  u  broken,"  the  ink  is  in  lines  or  dots,  and  it  is 
sometimes  necessary  to  look  at  the  print  from  a  little 
distance  so  that  the  lines  or  dots  may  merge  into  the  inter¬ 
spaces  and  give  the  appearance  of  a  continuous  tint.  It  is 
possible  to  make  the  patches  of  ink  so  very  small  that  the 
closest  inspection  will  not  reveal  them  :  in  such  cases  it 
only  needs  a  magnifying  glass  or  a  low  power  microscope 
to  show  that  they  are  really  present. 

If  the  subject  from  which  the  plate  is  to  be  made  is  a 
line  subject,  such  as  a  drawing  in  pen  and  ink  or  an  im¬ 
pression  from  a  woodcut,  the  photographic  production  of 
the  printing  plate  is  simple  in  theory  and  fairly  simple  in 
practice,  for  in  this  case  there  are  no  half  tones  to  trouble 
about.  We  will  suppose  that  it  is  desired  to  produce  a 
block  to  print  with  type  that  shall  represent  a  diagram 
drawn  in  black  lines  on  white  cardboard.  A  sheet  of  zinc 
with  a  flat  and  suitable  surface  is  coated  with  a  thin  film  of 
albumen  (white  of  egg)  made  sensitive  to  light  by  adding 
to  it  a  solution  of  ammonium  bichromate.  The  film  is 
dried  in  the  dark,  pressed  into  close  contact  with  a  nega¬ 
tive  made  from  the  original  drawing,  and  then  exposed  for 
a  short  time  to  a  strong  light.  Where  the  light  acts  upon 
the  film  the  albumen  is  made  insoluble,  and  it  might  be 
thought  that  it  only  remained  to  wash  away  the  soluble 
albumen  and  put  the  plate  into  acid  to  dissolve  away  the 
surface  of  the  exposed  metal,  to  get  the  plate  with  the 
exposed  lines  raised  on  it  ready  for  printing.  1  his  simple 
method  would  fail,  because  the  insoluble  albumen  would 
not  prove  an  effective  protection  to  the  zinc  in  the 


Photo-mechanical  Printing 

presence  of  the  acid.  The  exposed  plate  is  therefore  first 
coated  with  a  greasy  (printers’)  ink  by  means  of  a  roller, 
then  soaked  in  water  and  gently  rubbed  until  the  soluble 
parts  of  the  albumen  with  the  ink  on  them  are  washed 
away.  We  have  now  the  lines  in  insoluble  albumen  with 
a  thin  coating  of  a  greasy  ink  on  them.  By  sprinkling 
finely  powdered  bitumen  over  the  plate  it  will  adhere  to 
the  greasy  surface  of  the  lines  and  may  be  dusted  off  the 
plain  metal  surface,  and  when  gently  warmed  the  bitumen 
will  melt  with  the  ink  and  afford  a  sufficient  piotection  to 
the  metal  to  allow  of  a  preliminary  attack  with  acid.  But 
for  further  etching  the  lines  need  still  more  protection,  so 
the  washed  plate  is  sponged  over  with  a  solution  of  gum, 
dried,  and  more  ink  is  applied  by  the  roller.  The  plate 
may  now  be  returned  to  the  acid  that  a  little  more  of  its 
surface  may  be  dissolved  away  leaving  the  lines  standing 
up  in  higher  relief.  But  this  action  cannot  be  pushed  far 
or  the  acid  would  dissolve  the  sides  of  the  projecting  lines 
of  metal,  and  in  time  undercut  and  actually  detach  them. 
To  prevent  this  a  resinous  substance  is  dusted  on,  it 
adheres  to  the  greasy  ink,  and  by  gently  heating  the  plate, 
it  is  melted  and  allowed  to  flow  over  onto  the  sides  of  the 
projecting  lines  of  metal  and  protect  them.  By  continuing 
these  operations,  the  metal  is  gradually  dissolved  away 
where  it  is  not  protected,  until  the  lines  stand  up  suffi¬ 
ciently  high  to  allow  of  the  plate  being  printed  from. 

If  the  object  or  picture  of  which  a  printing  block  is  re¬ 
quired  does  not  consist  of  simple  black  lines  but  of  con¬ 
tinuous  tones  of  different  darknesses,  then  a  necessary 
preliminary  to  the  making  of  the  block  is  the  translation  o 
the  even  tones  into  an  equivalent  mixture  of  simple  black 
and  white.  Suppose,  for  example,  that  there  is  a  grey 
mid  wav  between  black  and  white,  we  must  have,  instead 
of  the  continuous  grey,  a  block  that  will  print  black  lines 
or  dots  that  shall  collectively  cover  say  half  the  space  tha 
represents  the  grey,  so  that  when  the  print  is  viewed  from 

219 


Photo-mechanical  Printing 

a  suitable  distance  the  black  dots  or  lines  and  the  white 
spaces  between  them  will  mingle  and  appear  like  the  grey 
that  is  desired.  If  the  grey  is  darker,  the  black  dots  or  lines 
must  be  thicker  or  more  numerous  so  that  there  is  more 
black  and  less  white,  and  if  the  grey  is  lighter  they  must  be 
thinner  or  less  numerous.  In  mechanical  methods  as 
distinguished  from  hand  engraving,  dots  are  almost  always 
used,  and  as  the  number  of  them  is  fixed,  the  problem 
resolves  itself  into  the  getting  of  larger  dots  or  smaller  dots 
to  represent  the  darker  and  the  lighter  tones  respectively. 
If  the  dots  are  of  a  regular  shape,  as  they  increase  in  size 
they  will  eventually  touch  each  other,  and  instead  of  a 
white  surface  with  black  dots  on  it,  there  will  be  a  black 
surface  with  white  dots.  These  results  are  simply  two 
phases  of  the  same  effect.  These  effects  will  be  easily 
traced  in  the  reproductions  from  the  photograph  of  a 
child.  The  second  has  dots  that  are  very  large  that  their 
character  may  be  clearly  seen,  but  even  this,  if  looked  at 
from  a  sufficient  distance,  will  show  the  picture  well. 

There  have  been  many  methods  proposed  for  making 
this  translation  of  true  half-tone  into  black  and  white.  It 
is  easy  to  imagine  a  carbon  print  made  with  a  white  pig¬ 
ment,  the  original  being  represented  by  a  relief  that  is 
higher  in  proportion  to  the  darkness  of  its  parts.  If  now  a 
kind  of  brush  with  short  india-rubber  taper  teeth  instead 
of  bristles  is  coated  with  black  ink  and  carefully  pressed 
against  the  white  print,  so  that  the  india-rubber  points  just 
touch  the  deepest  hollows  or  the  lowest  parts  of  the  prints, 
there  will  be  formed  in  these  lowest  parts  small  black  dots 
because  the  points  of  the  inked  india-rubber  teeth  will  just 
come  into  contact  with  the  surface.  But  in  those  parts 
that  stand  up  in  higher  relief  the  india-rubber  teeth  will  be 
flattened  against  the  surface  and  form  larger  dots  of  ink, 
and  thus  the  sizes  of  the  dots  will  vary  exactly  in  propor¬ 
tion  to  the  height  of  the  relief  and  therefore  in  proportion 
to  the  darkness  of  the  part  of  the  original  represented.  The 

220 


Reproduction  of  the  Photograph  of  a  Child 
with  a  Fine  Screen 

In  making  a  printing  block  from  a  picture  by  photographic  means,  a  photograph 
is  made  through  a  screen  that  is  covered  with  small  transparent  spaces  in  an  other- 
wise  opaque  ground.  This  gives  the  subject  in  equally  distanced  dots,  and  the  light 
and  shade  are  produced  by  the  variations  in  the  sizes  of  the  dots. 


Another  Reproduction  of  the  same  Photograph 
with  a  Coarse  Screen 

The  dots  in  this  print  are  so  large  that  they  are  more  visible  than  the  subject, 
twelve  to  the  linear  inch,  while  on  the  opposite  page  they  are  so  small  as  to  be 
invisible  to  the  naked  eye,  there  being  175  to  the  linear  inch.  By  looking  at  this 
print  from  a  sufficient  distance  the  dots  become  less  obtrusive,  and  the  subject  will 
be  clearly  seen. 


Photo-mechanical  Printing 

various  continuous  tones  have  thus  been  translated  into  a 
simple  “  black  and  white.” 

The  method  that  is  used  at  the  present  almost  to  the 
exclusion  of  others  is  not  quite  so  easy  to  understand  as 
that  just  described  and  others  of  a  similar  character, 
although  it  is  as  effective  and  from  a  practical  point  of 
view  superior.  An  ordinary  photographic  print  of  the 
object  is  photographed  through  a  cross-lined  screen  placed 
just  in  front  of  the  plate.  The  screen  is  covered  with 
opaque  lines  ruled  with  mechanical  regularity  and  in  two 
directions,  with  a  space  between  each  line  and  the  next 
about  equal  to  the  thickness  of  the  lines,  so  that  it  may  be 
regarded  as  an  opaque  screen  with  small  transparent 
squares  about  the  size  of  pinholes  regularly  disposed  over 
it.  For  many  years  it  was  not  clear  why  such  a  procedure 
as  this  should  give  the  desired  result,  and  while  allowing 
that  the  action  of  the  screen  is  complicated,  there  is  little 
doubt  now  as  to  its  primary  action.  At  the  same  time  that 
the  lens  gives  an  image  of  the  picture  on  the  plate  through 
the  screen,  each  hole  in  the  screen  gives  an  image  of  the 
lens  or  its  diaphragm  on  the  plate  after  the  manner  in 
which  a  pinhole  will  give  an  image,  as  we  have  seen  in  an 
earlier  chapter.  But  as  the  apertures  of  the  screen  are 
large  in  comparison  with  the  image  of  the  lens  that  they 
give,  the  little  patch  of  light  that  falls  upon  the  plate 
behind  each  aperture  in  the  screen  is  brightest  at  its  centre 
and  gradually  fades  off  towards  its  margins.  Where  there 
is  very  little  light,  that  is  corresponding  to  the  dark  parts 
of  the  subject,  there  will  only  be  enough  light  at  the  very 
centre  of  the  patch  to  give  an  image  on  development,  while 
where  a  brighter  light  from  the  subject  falls  upon  the  plate 
the  developable  patch  or  spot  will  be  largei.  Thus  the 
translation  of  the  tones  into  dots  of  various  sizes  is  effected, 
but  the  dots  have  margins  that  gradually  get  thinner  and 
are  therefore  indecisive.  The  dots  are  made  “  cleaner 
or  more  precise  by  applying  a  solvent  of  the  image  to  the 

221 


Photo-mechanical  Printing 

plate  which  removes  the  thin  edges  without  materially 

affecting  the  denser  central  parts. 

There  are  many  matters  that  have  to  be  taken  into 
consideration  in  settling  the  coarseness  of  the  screen,  or 
in  other  words  the  number  of  dots  in  a  straight  line  that 
would  be  included  within  a  measured  inch.  For  printing 
on  a  rough  surface  paper,  such  as  is  used  for  newspapers, 
and  especially  when  the  printing  has  to  be  rapidly  done, 
the  screen  must  be  coarse  so  that  the  spaces  between  the 
dots  may  be  comparatively  large  and  stand  a  deep  etching 
to  prevent  them  from  getting  filled  up  with  ink.  But  large 
dots  are  obtrusive  and  obliterate  detail,  and  from  this 
point  of  view  the  finer  the  dots  the  better.  To  get  the 
best  results  from  blocks  made  with  a  very  fine  screen  the 
surface  of  the  paper  must  be  very  smooth  and  the  printing 
very  carefully  done.  About  sixty  dots  to  the  inch  is  suit¬ 
able  for  a  common  paper,  but  a  person  with  ordinary 
sight  will  see  such  dots  when  the  print  is  held  at  the  best 
distance  for  distinct  vision.  With  one  hundred  and  twenty 
to  the  inch  the  dots  would  be  invisible  unless  the  print 
were  brought  closer  to  the  eye,  and  from  this  to  about  a 
hundred  and  fifty  is  commonly  used  for  book  illustrations. 
If  very  minute  detail  has  to  be  reproduced  the  fineness 
may  increase  up  to  two  hundred,  or  in  exceptional  cases 
to  four  hundred  to  the  linear  inch.  With  this  last  most 
persons  would  fail  to  see  the  dots  even  when  their  sight 
was  assisted  with  a  watchmaker’s  magnifying  glass. 

For  making  the  printing  surface  from  the  negative 
taken  through  a  cross-lined  screen,  a  sheet  of  copper  of  the 
required  size  and  with  a  polished  surface  is  coated  with  a 
solution  of  “  fish  glue  "  containing  ammonium  bichromate 
to  render  it  sensitive.  The  commonest  equivalent  of  fish 
glue  is  “seccotine,”  which  is  well  known  to  resemble  a 
gum  mucilage  in  remaining  liquid  when  cold.  The  fish 
glue  used  is  specially  filtered  to  ensure  a  clean  coating. 
The  plate  is  gently  warmed  to  dry  the  film,  which  is  very 

222 


Photo-mechanical  Printing 

thin,  and  then  exposed  to  light  under  the  negative.  After 
exposure  it  is  washed  that  the  parts  not  changed  by  light 
may  be  dissolved  away,  and  as  what  remains  is  hardly 
visible,  being  thin  and  colourless,  the  plate  is  put  into  a 
dye  solution  which  stains  the  image  and  serves  to  show 
whether  it  is  satisfactory.  If  all  is  well  the  plate  is  then 
heated  to  a  pretty  high  temperature,  hot  enough  to  burn 
wood  but  not  sufficiently  hot  to  char  the  fish  glue.  This 
changes  the  coating  into  a  very  hard  enamel-like  substance, 
which  will  not  only  protect  the  metal  during  the  etching, 
but  is  so  much  more  able  to  resist  the  wear  of  printing  than 
the  copper  itself,  that  it  may  be  intact  after  sixty  thousand 
impressions  have  been  taken  under  conditions  that  ten 
thousand  impressions  would  notably  deteriorate  the  block 
if  of  bare  copper,  and  this  although  the  film  is  as  thin  as 
a  soap  bubble  just  before  it  bursts.  To  etch  the  plate  it  is 
put  into  a  solution  of  ferric  chloride,  that  is  a  compound 
of  iron  with  the  maximum  amount  of  chlorine,  and  which 
in  the  presence  of  copper  gives  upborne  of  its  chlorine  to 
it,  and  the  chloride  of  iron  that  remains  as  well  as  the 
chloride  of  copper  that  is  produced  can  be  washed  away. 
The  plate  now  only  needs  to  be  mounted  on  a  suitable 
wooden  block  to  be  ready  for  the  printing  press,  but  the 
simple  process  here  described  is  often  supplemented  by  a 
little  further  etching  of  some  parts  or  a  little  touching  up 
by  hand. 

Those  illustrations  that  used  to  be  called  “copper 
plates  ”  or  “  steel  plates,"  meaning  impressions  taken  from 
copper  or  steel  plates  on  which  the  picture  was  engraved, 
were  produced  by  inking  the  plate  all  over,  wiping  the 
surface  of  the  plate  clean  so  that  the  ink  remained  only 
in  the  cut  out  lines  and  dots,  and  then  taking  the  im¬ 
pression.  These  processes  are  now  almost  obsolete  and 
their  modern  representative  is  called  “  photogravure. 
The  polished  copper  plate  for  this  process  has  to  have  the 
picture  put  upon  it  as  a  “  resist,"  and  it  is  then  etched  to 

223 


Photo-mechanical  Printing 

form  the  depressions  that  are  to  hold  the  ink.  The  resist 
is  a  carbon  print,  and  that  as  we  know  consists  essentially 
of  a  film  of  gelatine  that  is  graduated  in  thickness  accord¬ 
ing  to  the  darkness  of  the  various  parts  of  the  original.  But 
as  in  this  case  where  the  most  etching  takes  place  the  most 
ink  will  be  held  and  there  will  be  the  darkest  part  of  the 
impression,  we  want  the  thinnest  part  of  the  carbon  print 
to  correspond  with  the  darkest  part  of  the  subject,  and 
the  thickest  part  of  the  print  with  the  lightest  part  of  the 
subject— that  is,  the  carbon  print  must  be  a  negative 
instead  of  as  usual  a  positive.  The  common  method  is  to 
prepare  from  the  ordinary  negative  a  carbon  transparency, 
which  is  a  positive,  and  from  this  to  make  another  carbon 
print  which  will  be  the  required  negative  and  will  be 
mounted  on  the  copper  plate  for  development.  If  this 
were  done  and  the  plate  etched,  the  gelatine  film  which 
constitutes  the  carbon  print  acting  as  a  resist  and  regu¬ 
lating  the  extent  of  the  etching  in  the  various  parts  of  the 
plated  the  image  would  be  etched  on  the  plate  as  a  very 
shallow  relief,  but  one  which  would  not  hold  ink  when  the 
plate  was  wiped,  for  the  ink  would  be  wiped  out  from  the 
shallow  depressions  just  as  it  might  be  wiped  out  from  the 
inside  of  a  saucer.  As  explained  in  connection  with  the 
previous  process  the  general  surface  of  the  metal  must 
be  retained,  and  etching  restricted  to  very  small  areas 
of  it,  forming  minute  depressions  or  pits  which  will  hold 
the  ink.  The  “grain”  necessary  to  produce  this  result 
is  obtained  by  covering  the  surface  of  the  copper  plate, 
after  it  has  been  well  polished,  with  minute  particles  of 
bitumen  or  resin,  by  shaking  up  the  powdered  material 
in  a  closed  box,  allowing  it  to  stand  for  a  short  time 
that  the  coarser  particles  may  settle,  and  then  putting  the 
plate  in  that  the  finer  particles  may  rain  down  or  sub¬ 
side  upon  it.  By  gently  heating  the  plate,  each  little 
particle  melts  to  a  drop,  and  protects  that  portion  of  the 
surface  that  it  covers  from  the  subsequent  etching.  I  he 


Photo-mechanical  Printing 

**  ground  ”  may  be  laid  by  other  means,  as  for  example  by 
spraying  a  solution  of  the  substance  upon  the  plate,  but 
the  result  obtained  is  much  the  same.  Upon  the  plate 
so  prepared,  the  carbon  print  already  described  is 
mounted  and  developed  in  the  ordinary  manner,  and 
when  dry  the  plate  is  ready  for  the  etching.  A  solution 
of  ferric  chloride  is  generally  used,  and  its  action,  which 
takes  but  a  short  time,  can  be  followed  by  the  change  of 
colour  that  it  produces  of  the  metal  surface.  Where  the 
gelatine  film  of  the  carbon  print  is  thin,  that  is  in  the 
dark  parts  of  the  subject,  the  etching  begins  first  and 
therefore  finally  produces  the  greatest  effect,  and  as  soon 
as  the  etching  has  begun  where  the  thickest  parts  of  the 
gelatine  film  lie,  corresponding  with  the  brightest  parts 
of  the  subject,  the  plate  is  washed  to  stop  the  action,  and 
the  gelatine  and  bitumen  are  cleaned  off  by  brushing  with 
a  solution  of  carbonate  of  soda.  The  plate  is  now  ready 
to  be  printed  from,  unless  it  needs  any  touching  up,  but 
if  many  impressions  are  wanted  it  is  usual  to  “steel  face” 
it,  by  electrically  depositing  upon  it  a  very  thin  film  of  the 
harder  metal.  The  etching  is  of  a  very  different  character 
from  that  described  in  connection  with  other  processes, 
indeed  it  may  be  more  graphically  described  as  a  mere 
roughening  of  the  surface  of  the  metal.  The  roughening 
is  more  extensive  and  deeper  where  the  etching  liquid 
acts  for  the  longest  time,  and  in  these  parts  when  the 
plate  is  inked  and  wiped  ready  for  taking  the  impression, 
the  ink  that  is  retained  not  only  covers  a  greater  pro¬ 
portion  of  the  surface  of  the  plate,  but  is  held  in  greater 
quantity  in  the  slightly  deeper  depressions.  The  gradation 
in  the  print  depends  upon  both  these  circumstances. 

It  must  not  be  supposed  that  all  the  applications  of 
photography  to  mechanical  printing  processes  have  been 
described  or  even  referred  to  in  this  chapter,  for  we  have 
only  aimed  at  giving  the  general  lines  upon  which  such 
applications  are  worked  out.  Each  fundamental  process 


Photo-mechanical  Printing 

is  subject  to  numerous  variations  to  adapt  it  to  the  different 
conditions  of  the  many  sorts  of  subjects  that  have  to  be 
dealt  with,  and  to  the  results  required.  For  example, 
photogravure,  which  seems  so  essentially  a  hand  process 
and  more  suitable  for  the  artist  himself  than  for  the  mere 
printer,  has  been  adapted  to  rapid  printing  by  rotary 
presses.  The  subject  of  photo-mechanical  printing  is  very 
wide  and  very  technical,  and  will  be  briefly  referred  to 
again  in  connection  with  the  photography  of  colour. 

As  an  addendum  to  this  chapter  we  may  perhaps 
refer  to  a  matter  that  seems  to  demand  reference,  though 
only  a  brief  reference,  because  while  it  concerns  photo¬ 
graphs  it  is  not  a  photographic  but  an  electrical  subject, 
namely  the  transmission  of  photographs  by  means  of 
telegraphy.  When  we  say  that  it  is  possible  to  send  a 
photograph  of  a  person  or  of  an  event  in  ten  or  fifteen 
minutes  from  Paris  or  Manchester  to  London  or  even 
from  a  much  greater  distance,  the  importance  of  the 
achievement  may  be  realised.  It  means  that  photographs 
can  arrive  from  a  distance  a  day  earlier  than  if  they  had 
to  be  carried  by  messenger,  and  this  day  saved  may 
make  all  the  difference  between  success  and  failure,  as 
in  transmitting  the  portrait  of  a  criminal  or  the  repre¬ 
sentation  of  an  event  of  importance.  In  1907,  the  Daily 
Mirror  in  London,  L Illustration  in  Paris,  and  the  Lokal 
Anzeiger  in  Berlin  installed  apparatus  designed  for  this 
purpose  by  Professor  A.  Korn,  and  a  year  later  one  was 
set  up  in  Manchester.  Since  then  the  telegraphic  trans¬ 
mission  of  photographs  has  been  a  regular  practice  for 
publishing  purposes,  and  many  improvements  have  been 
effected,  notably  by  Mr.  Thorne  Baker. 

There  is  only  one  current  of  electricity  passing  between 
the  transmitting  and  the  receiving  station,  and  all  that 
can  be  done  with  it  is  to  increase,  diminish,  stop,  or 
start  it.  It  is  out  of  the  question  to  deal  with  the  picture 
as  a  whole,  and  it  is  necessary  to  “  telegraph  ”  as  it  were 

226 


Photo-mechanical  Printing 

just  one  small  part  of  it  at  a  time.  To  enable  this  to 

be  done  a  suitable  copy  of  the  picture  is  bent  upon 

and  round  a  cylinder  which  revolves  and  moves  very 
slightly  longitudinally,  so  that  a  fixed  point  held  against 
it  would  make  a  long  spiral  line  that  would  cover 
practically  the  whole  surface  of  the  picture.  By  this 
continuous  movement  every  part  of  the  picture  is  brought 
in  turn  to  the  same  place,  and  if  the  cylinder  is  of  metal 
and  the  picture  is  prepared  on  a  sheet  of  metal  foil  with  a 
material  that  will  not  allow  an  electric  current  to  pass, 
then  as  the  cylinder  revolves  the  current  passes  where 
the  metal  foil  is  bare,  but  whenever  a  part  of  the  drawing 
comes  beneath  the  point  the  current  is  stopped.  The 

receiver  consists  of  a  similar  cylinder  covered  with  a 

sheet  of  paper  that  is  so  prepared  that  when  an  electric 
current  passes  through  it  a  dark  deposit  is  produced.  1  he 
two  cylinders  are  made  to  revolve  at  exactly  the  same 
rate,  and  thus  all  the  spaces  of  bare  metal  in  the  drawing 
are  represented  at  the  distant  cylinder  by  the  deposit, 
and  if  the  original  is  a  negative,  the  distant  reproduction 
is  a  positive.  There  are  other  ways  that  serve  to  translate 
the  light  and  shadow  of  a  picture,  piecemeal,  into  varia¬ 
tions  of  an  electric  current,  and  from  the  varying  current 
to  reproduce  the  picture,  but  the  main  principle  involved 
is  always  analogous  to  that  described. 


227 


CHAPTER  XIV 

THE  EFFECT  OF  COLOUR  AND  ITS  CONTROL 

Up  to  this  point  in  the  consideration  of  our  subject  we 
have  ignored  colour,  treating  of  the  photography  of 
objects  as  if  they  were  all  white,  black,  or  of  the  various 
intermediate  shades  of  grey.  But  one  has  only  to  look 
around,  whether  he  is  indoors  or  out-of-doors,  in  the 
town  or  the  country,  to  be  convinced  that  almost  every¬ 
thing  is  coloured,  and  that  pure  blacks,  whites  and  greys 
are  very  rare.  In  the  house,  not  only  the  curtains,  carpets 
and  tapestries  have  their  own  colours,  but  the  wood  of 
the  furniture  and  the  brass  of  the  fittings  form  no  ex¬ 
ception  to  the  rule ;  and  out-of-doors,  the  houses,  the 
roads,  the  sea,  and  the  sky,  and  almost  all  animals,  plants 
and  flowers,  are  coloured.  We  will  endeavour  to  find 
some  of  the  effects  of  colour  in  photography,  the  reasons 
for  them,  and  the  methods  by  which  they  can  be  con¬ 
trolled. 

Assuming  that  the  photographer  employs  an  ordinary 
photographic  plate  and  takes  no  precautions  concerning 
colour,  and  that  is  the  condition  under  which  nine-tenths 
if  not  ninety-nine  hundredths  of  the  photography  of 
the  present  day  is  carried  out,  he  is  likely  to  be  surprised 
by  some  remarkable  results.  He  may  have  become  so 
accustomed  to  getting  no  ‘‘sky"  in  his  negatives,  that  he 
may  not  notice  when  the  blue  sky  and  the  white  clouds 
produce  the  same  effect  upon  his  plate,  so  that  the 
difference  between  them  is  entirely  lost ;  but  when  a 
child’s  coat  that  is  of  a  bold  red  and  blue  plaid  shows 


The  Effect  of  Colour  and  its  Control 

in  the  photograph  as  if  it  has  no  pattern  at  all  upon  it, 
the  shortcomings  of  his  method  are  forced  upon  his 
attention.  Other  cases  might  be  described  where  the 
detail  due  to  colour  is  lost.  The  reverse  is  also  possible, 
for  the  exaggeration  of  the  appearance  of  yellow  freckles 
in  the  skin,  which  may  be  hardly  noticeable  in  the  person 
but  photograph  as  if  they  were  dark  spots,  was  one  of 
the  earliest  reasons  that  led  portrait  photographers  to 
correct  their  negatives  by  working  upon  them  with  a 
pencil,  brush,  or  knife.  The  observation  of  these  and 
other  discrepancies  due  to  colour,  were  the  cause  of 
many  leaflets  of  “advice  to  sitters”  concerning  the  colour 
of  the  clothes  that  would  give  the  most  pleasing  results. 
All  such  advice  was,  and  so  far  as  it  exists  still  is,  a 
confession  of  inability  on  the  part  of  the  photographer 
to  represent  certain  colours  as  they  appear  to  the  eye. 
Yellows  would  come  out  too  dark,  some  reds  darker  still, 
blues  often  too  light,  greens  too  dark,  so  that  error  was 
universal.  It  is  true  that  people  got  used  to  seeing  grass 
represented  in  landscape  photographs  as  much  darker 
than  it  really  is  to  the  eye,  and,  as  the  appreciation  of 
oak  and  mahogany  articles  generally  increases  as  the 
woods  darken,  there  might  be  no  objection  to  them  ap¬ 
pearing  in  the  picture  as  several  shades  darker  than  they 
were  in  reality.  But  no  one  appreciated  the  representation 
of  a  red  rose  as  if  it  were  black,  a  buttercup  as  if  it  were 
a  dark  grey,  and  a  violet  as  if  it  were  white. 

The  colour  of  every  object  is  due  to  two  circum¬ 
stances  ;  first,  the  light  that  shines  upon  it,  and  second, 
the  effect  that  the  object  has  upon  the  light.  We  too 
often  consider  the  second  circumstance  only,  but  a  little 
thought  will  call  to  mind  the  importance  of  the  first. 
The  matching  of  colours,  if  they  are  required  to  appear 
the  same  by  daylight,  cannot  be  done  by  artificial  light, 
and  paintings  made  by  daylight  are  falsified  when  illu¬ 
minated  by  other  means.  But  surely,  some  may  say,  a 

229 


The  Effect  of  Colour  and  its  Control 

red  rose  is  always  red  and  a  buttercup  is  always  yellow. 
But  it  is  not  so,  and  without  going  to  the  extreme  of 
pointing  out  that  neither  is  coloured  in  the  dark,  although 
this  is  true,  because  it  might  be  argued  that,  as  we  cannot 
see  anything  in  the  dark,  the  red  and  yellow  may  still 
be  there  so  far  as  we  can  tell,  so  simple  an  experiment 
as  looking  at  the  flowers  by  the  yellow  light  produced 
by  putting  a  little  common  salt  in  a  spirit  lamp  flame, 
will  show  that  the  colour  depends  not  only  upon  the 
object  but  also  upon  the  light  that  shines  upon  it.  In 
this  light,  yellow  will  appear  to  be  white,  while  a  pure 
red  or  a  pure  blue  or  a  deep  green  will  appear  to  be 
black.  Those  who  are  interested  in  the  experiment  and 
have  no  special  facilities  for  performing  it,  may  crush 
or  grind  some  table  salt  as  finely  as  possible,  stir  up 
some  of  it  in  a  saucer  with  a  little  spirit  of  wine  or  methyl¬ 
ated  spirit  and  set  fire  to  the  mixture.  An  iron  skewer 
would  serve  for  stirring  it  up  to  get  a  brighter  flame,  and 
the  flame  may  be  extinguished  by  putting  a  piece  of 
card  or  an  old  book  on  the  top  of  the  saucer.  The 
altered  appearance  of  almost  any  coloured  article  when 
illuminated  by  this  flame  will  show  that  the  colour  de¬ 
pends  upon  the  character  of  the  light  that  renders  the 
object  visible. 

We  saw  in  the  first  chapter  that  ordinary  light  is 
not  homogeneous,  but  is  a  mixture  of  many  kinds  of 
vibrations.  These  vibrations  may  be  compared  to  the 
waves  or  ripples  on  a  water  surface,  although  they  are 
not  on  a  surface  but  in  the  bulk  of  the  luminiferous 
ether,  and  in  order  to  distinguish  them  it  is  usual  to 
refer  to  the  wave  length  of  each,  that  is,  the  distance 
from  the  crest  of  one  wave  to  the  crest  of  the  next. 
Sound  is  also  a  wave  movement,  but  of  a  rather  different 
kind,  and  takes  place  in  the  air  instead  of  in  the  ether. 
If  one  note  of  the  piano  is  struck,  there  results  a  sound 
that  is  generally  referred  to  as  having  a  constant  and 

230 


The  Effect  of  Colour  and  its  Control 

single  wave  length,  though  it  is  not  really  quite  so  simple. 
But  if  now  all  the  notes  that  are  included  in  some  three 
or  four  octaves  were  struck  at  the  same  time  we  should 
get  a  terribly  discordant  and  complex  mixture  of  sounds 
of  various  pitches,  that  is,  of  various  wave  lengths.  Now 
white  light,  or  ordinary  daylight,  is  a  very  much  more 
complex  mixture  of  lights  than  the  noise  just  described 
is  of  sounds.  It  is  so  very  much  more  complex  that 
it  is  impossible  to  suggest  any  figure  that  shall  even 
roughly  indicate  how  much  more  complex  it  is.  Instead 
of  about  three  dozen  notes,  each  having  its  chaiacteristic 
wave  length,  we  have  in  ordinary  light  an  inestimable 
number  of  thousands  of  wave  lengths,  so  that  it  is  im¬ 
possible  to  attempt  to  picture  the  confusion. 

There  are  different  ways  by  which  these  may  be  more 
or  less  separated  from  one  another.  The  simplest  is  by 
getting  a  thin  band  of  light  by  allowing  it  to  shine  through 
a  narrow  slit,  as  narrow  perhaps  as  the  thickness  of 
a  sheet  of  paper,  and  then  bending  this  band  of  light 
as  one  might  bend  a  strip  of  paper.  The  bending,  of 
course,  is  not  done  with  the  fingers,  but  by  allowing  the 
light  to  pass  through  a  wedge  or  prism  of  glass.  The 
same  prism  or  battery  of  prisms  bends  the  various  con¬ 
stituents  of  light  to  different  extents,  so  that  instead  of 
getting  a  simple  line  of  light  on  the  screen,  the  line  is 
thickened  or  spread  out  according  to  the  power  of  the 
apparatus  until  it  may  become  a  band  several  feet  long. 
No  one  has  yet  spread  it  out  to  such  an  extent  as  to  get 
a  separation  of  the  individual  wave  lengths  of  ordinary 
white  light,  but  the  waves  of  shorter  length  are  more 
bent,  under  the  same  conditions,  than  the  waves  o 
greater  length,  and  the  proportional  bending  or  sprea  ing 
out  is  regular.  A  band  of  light  so  produced  is  called  a 
spectrum,  and  in  this  way  it  is  possible  to  analyse  any 
light,  artificial  or  natural,  and  compare  the  constituents 

or  wave  lengths  of  various  lights. 

231 


The  Effect  of  Colour  and  its  Control 

The  most  complex  light  gives  a  spectrum  that  may 
from  one  point  of  view  be  regarded  as  the  simplest, 
because  it  is  a  complete  band  with  no  breaks  in  it.  Only 
a  part  in  the  middle  of  such  a  spectrum  affects  the 
human  eye  and  is  what  we  call  in  common  language, 
light,  and  of  this  visible  part  the  portion  least  bent  is 
red,  and  then  follow  in  regular  order  orange,  yellow, 
green,  blue,  and  the  portion  most  bent  is  violet,  as  already 
shown  in  Fig.  5*  Beyond  the  red  is  the  infra-red, 
and  after  the  violet  is  the  li  ultra-violet,”  and  to  these 
the  normal  human  eye  is  blind.  A  colour  blind  person 
is  one  who  sees  some  of  these  colours  imperfectly,  and 
he  is  called  colour  blind  because  his  power  of  vision  is 
restricted  as  compared  with  human  eyes  in  general.  If 
we  took  the  sum  total  of  light  as  our  standard,  every 
one  would  have  to  be  regarded  as  very  colour  blind 
indeed. 

Considering  this  complete  gamut  of  light,  we  may  say 
that  almost  everything,  like  our  eyes,  is  affected  by  only 
a  part  of  it,  or  differently  by  different  parts  of  it.  A  red 
object,  if  it  is  purely  red,  is  red  because  it  takes  out  of 
the  light  that  shines  upon  it  all  the  light  that  we  can 
see  except  the  red,  and  this  it  reflects.  Whether  it 
absorbs  or  reflects  the  infra-red  or  the  ultra-violet  would 
not  affect  the  colour  to  our  eyes,  because  these  are  in¬ 
visible,  and  therefore  our  eyes  cannot  distinguish  between 
their  presence  and  their  absence.  In  a  similar  way  a 
blue  object  appears  blue  because  it  absorbs  or  quenches 
all  the  colours  except  the  blue  and  reflects  this.  And 
so  on  with  other  colours,  while  a  black  object,  so  far 
as  it  is  black,  absorbs  all  the  colours,  and  a  white  object 
reflects  all. 

A  similar  set  of  conditions  apply  to  transparent  media. 
A  piece  of  colourless  glass  is  colourless  because  it  allows 
all  the  visible  light  that  falls  upon  it  to  pass  through  it, 
while  a  red  glass  is  red  because  while  it  allows  red  to 

232 


The  Effect  of  Colour  and  its  Control 

pass  through  it,  it  stops  or  absorbs  the  green  and  the 
blue.  We  may  say  of  any  piece  of  coloured  glass  that 
it  is  transparent  to  its  own  colour  and  opaque  to  the 
other  colours,  and  when  used  for  stopping  or  reducing 
the  intensity  of  a  part  of  the  light,  allowing  the  rest  to 
pass  freely  through  it,  such  a  glass  is  called  a  colour 
filter  or  colour  screen,  because  it  filters  out  some  of  the 
light  and  allows  the  rest  to  pass  on,  and  so  may  be 
compared  to  a  sieve  or  screen  that  allows  the  sand  to 
pass  through  it  while  it  retains  the  stones. 

We  are  now  prepared  to  understand  how  the  nature 
of  the  light  affects  the  colour  of  the  object.  A  red 
object  is  red  because  it  reflects  red  light  only,  but  if  we 
cause  the  white  light  to  pass  through  a  colour  filter  that 
stops  or  absorbs  that  constituent  of  light  that  appears  red, 
using  for  example  a  blue  glass,  then  the  object  that  is 
red  in  white  light  will  appear  black  in  this  light,  because 
it  cannot  reflect  any  other  colour  than  red  and  here  there 
is  no  red  for  it  to  reflect.  But  if  we  use  a  red  light 
filter,  then  the  red  object  will  appear  like  a  white  object, 
because  both  the  red  and  the  white  object  can  reflect 
the  red  light  that  falls  upon  them,  and  the  white  can  re¬ 
flect  no  more  than  the  red,  because  the  other  constituents  of 
the  light  have  been  removed.  Thus  it  is  possible  to  make 
any  simple  colour  appear  either  black  or  white  to  the 
eye  if  we  have  the  opportunity  of  regulating  the  light 
that  falls  upon  it,  and  the  light  can  be  controlled  if 
suitable  light  filters  are  available.  And  this  control  can 
be  extended  in  an  exactly  similar  way  to  photography. 
A  buttercup,  for  example,  in  front  of  a  black  and  white 
screen,  can  be  photographed  so  that  the  flower  appears 
white  by  using  a  yellow  light  filter,  and  it  can  then  be 
photographed  so  that  it  appears  black  by  using  a  light 
filter  that  is  of  a  pure  blue,  the  light  filter  in  each  case 
being  conveniently  fixed  to  the  lens  so  that  only  the 
light  of  the  required  colour  passes  into  the  camera  to 

233 


The  Effect  of  Colour  and  its  Control 

act  upon  the  plate.  It  is  perhaps  obvious  that  if  it  is 
possible  to  get  a  light  filter  to  remove  practically  all  of 
a  certain  kind  of  light  that  is  not  wanted,  that  by  regulating 
the  intensity  of  the  colour  of  the  filter  it  is  possible  to 
remove  any  desired  proportion  of  any  part  of  the  light 
that  may  produce  too  much  effect.  In  this  way  the  photo¬ 
grapher  may  have  a  complete  control  over  the  character 
of  the  light  that  he  allows  to  act  upon  the  plate. 

We  have  referred  above  to  the  spectrum  as  a  complete 
gamut  of  light,  and  to  the  fact  that  only  the  central  part 
of  it  affects  the  eye  and  produces  the  sensation  that  we  call 
light.  It  is  not  the  same  part  of  it  that  is  most  active  in 
affecting  a  photographic  plate  so  that  the  silver  salt  that  it 
contains  is  changed  into  the  developable  condition.  And 
indeed,  however  many  different  sensitive  substances  that  we 
might  experiment  upon,  we  shall  probably  not  find  two  of 
them  exactly  alike  in  their  proportion  of  sensitiveness  to  the 
different  parts  of  the  spectrum.  We  may  say  as  a  rough 
approximation  that  the  notable  sensitiveness  of  an  ordinary 
photographic  plate  begins  in  about  the  middle  of  the  region 
of  the  spectrum  that  affects  the  eye,  and  extends  considerably 
past  the  blue  and  violet.  Yellow  and  red,  which  are  fairly 
bright  to  the  eye,  are  dark,  that  is  almost  like  black,  to  the 
photographic  plate,  for  it  is  practically  unaffected  by  them. 

It  has  been  said  that  therefore  the  photographic  plate 
is  colour  blind.  Both  the  words  “colour"  and  “blind” 
are  inapplicable  to  a  plate,  for  it  cannot  appreciate  colour 
nor  can  it  see,  but  if  the  term  is  to  be  used  in  the  sense 
in  which  it  is  here  intended,  we  may  truly  say  that  the 
plate  is  no  more  colour  blind  than  the  eye,  indeed  less 
so,  for  if  to  the  plate  red  and  yellow  are  dark,  to  the  eye 
the  violet  is  dark  and  the  large  extent  of  the  ultra-violet 
black,  and  here  the  photographic  plate  is  very  considerably 
sensitive.  The  simple  fact  is  that  the  eye  and  the  plate  are 
differently  sensitive. 

We  judge  of  colours  by  our  eyes,  and  we  want  pictures 

234 


The  Effect  of  Colour  and  its  Control 

to  represent  them  as  nearly  as  possible  as  we  see  them. 
To  the  eye  yellow  and  yellowish  green  are  the  brightest 
colours  and  red  is  rather  less  bright,  but  they  aie  almost 
like  black  to  the  plate,  while  blue,  which  produces  almost 
as  much  effect  as  white  upon  the  plate,  is  to  the  eye 
darker  than  either  red,  yellow,  or  green.  It  is  this  differ¬ 
ence  of  sensitiveness  that  accounts  for  all  the  discrepancy 
that  colour  causes  between  what  we  see  and  what  we  get 
in  a  photograph.  Now  an  ordinary  gelatino-bromide  plate 
is  not  altogether  insensitive  to  green,  yellow,  and  ted,  but 
its  sensitiveness  to  these  colours  is  so  small  that  it  is  quite 
inappreciable  under  ordinary  circumstances.  If  we  use 
a  deep  orange  colour  filter,  one  that  will  stop  or  filter  out 
about  nine  hundred  and  ninety-nine  thousandths  of  the 
violet  and  blue  light  and  altogether  stop  the  ultra-violet,  then 
we  give  red,  yellow,  and  green  a  chance  to  produce  an  effect 
upon  the  plate,  because  the  period  of  the  exposure  can  be 
increased  to  nearly  a  thousand  times  without  the  blue 
and  violet  producing  too  much  effect.  By  thus  handi¬ 
capping  the  light  that  is  excessively  active,  it  is  possible 
to  get  all  coloured  objects  represented  in  a  photograph 
with  very  much  the  same  relative  brilliancy  that  they 
appear  to  have.  This  increase  of  exposure  to  about  a 
thousand  times  is  quite  possible  with  some  subjects, 
especially  if  brilliant  out-of-door  daylight  is  available,  as 
it  is  in  some  countries  where  the  weather  is  more  reliable 
than  it  is  in  England.  But  in  the  great  majority  of  cases 
such  an  increase  in  the  period  of  the  exposure  is  quite  out 
of  the  question,  and  hence  the  increasing  of  the  sensitive¬ 
ness  of  the  plate  to  green,  yellow  and  red,  becomes  a 

matter  of  the  first  importance. 

Professor  Hermann  Vogel  of  Berlin,  in  1873,  found 
that  some  plates  were  abnormally  sensitive  to  green  light, 
and  was  thus  led  to  try  the  experiment  of  adding  dyes  to 
the  sensitive  salt,  hoping  that  a  dye  that  would  absorb 
green,  yellow,  or  red  light  would  increase  the  sensitiveness 

235 


The  Eftect  of  Colour  and  its  Control 

of  the  plate  to  the  colour  that  it  retained.  His  hopes  were 
realised,  and  he  prepared  plates  sensitive  to  green  light 
and  others  sensitive  to  red  according  to  the  dye  that  he 
employed.  He  found,  however,  that  the  sensitiveness  of 
the  plate  to  blue  was  still  proportionately  too  great,  and 
so  he  put  a  yellow  glass  in  front  of  his  lens  to  reduce  the 
power  or  intensity  of  the  blue.  In  these  experiments  Dr. 
Vogel  laid  the  foundation  of  what  is  called  isochromatic 
or  orthochromatic  photography,  but  as  the  modern 
gelatine  plate  was  not  known  at  that  time,  it  was  not  until 
rather  more  than  ten  years  after  this  that  isochromatic 
or  orthochromatic  plates  as  we  now  know  them  were 
introduced. 

An  “  orthochromatic  ”  or  “isochromatic"  plate  is, 
strictly  speaking,  one  that  will  render  coloured  objects 
so  that  in  the  print  they  appear  as  proportionately  bright 
or  dark  as  they  appear  to  the  eye,  irrespective  of  their 
colour.  But  as  these  terms  were  used  to  describe  the 
first  commercial  plates  that  constituted  a  step  in  this 
direction,  their  meaning  has  been  restricted  ever  since 
to  plates  that  have  an  enhanced  sensitiveness  to  green 
light,  red  light  remaining  almost  without  action  upon 
them.  If  the  plate  is  of  notable  sensitiveness  to  red  as 
well  as  to  green  it  is  called  “  panchromatic."  There  is 
no  plate  that  will  of  itself  render  various  shades  of  blue, 
green,  and  red  correctly,  because  the  sensitiveness  to 
blue  always  remains  largely  in  excess.  If  the  action  of 
the  blue  is  reduced  by  means  of  a  yellow  filter,  as  already 
explained,  it  is  possible  to  get  blues  and  greens,  almost 
without  exception,  represented  as  desired  by  using  an 
orthochromatic  plate,  but  yellows  are  only  partially 
corrected,  that  is  they  come  out  too  dark,  while  reds 
are  scarcely  at  all  improved.  By  using  a  darker  colour 
filter  it  is  possible  to  get  blue  and  yellow  relatively  correct, 
but  such  a  filter  will  over-correct  for  the  green,  making 
it  appear  too  light. 


236 


The  Effect  of  Colour  and  its  Control 

When  orthochromatic  plates  were  first  introduced, 
designs  consisting  of  various  combinations  of  blue  and 
yellow,  sometimes  with  white  and  black  included,  were 
offered  as  subjects  to  try  them  on.  Messrs.  B.  J.  Edwards 
&  Co.,  who  were  the  first  makers  of  orthochromatic  plates 
in  England,  printed  a  bright  yellow  disc,  with  the  letters 
“XL"  on  it  in  very  dark  blue,  and  “trade  mark"  in  white. 
The  yellow  was  so  light  that  the  white  “trade  mark" 
was  hardly  noticeable  until  searched  for,  and  the  blue 
was  almost  black,  so  that  what  appeared  to  the  casual 
observer  was  a  practically  black  “  XL "  on  a  light  yellow 
ground.  But  a  photograph  on  an  ordinary  plate  showed 
“  trade  mark "  as  if  it  were  printed  in  white  on  a  dark 
ground,  and  the  “  XL  "  did  not  appear,  for  the  dark  blue 
and  the  yellow  were  so  balanced  that  they  produced  the 
same  effect  in  the  plate.  This  experiment  was  very 
striking,  for  the  original  and  the  photograph  appeared 
to  be  quite  dissimilar.  By  means  of  an  orthochromatic 
plate  and  a  suitable  yellow  light  filter,  the  blue  could  be 
photographed  as  if  it  were  black  and  the  yellow  as  if  it 
were  white,  or  with  any  intermediate  effect.  By  using 
only  these  two  colours  the  advantage  of  the  plates  was 
demonstrated  without  their  shortcomings  being  manifest, 
for  if  a  good  yellowish  green  or  red  or  both  of  these  had 
been  added,  then  the  limits  of  their  power  for  colour 
correction  would  have  been  obvious.  If  by  the  use  of  a 
suitable  colour  filter  the  blue  and  the  yellow  had  been 
obtained  according  to  their  respective  darkness,  the  red 
would  have  shown  almost  as  if  it  were  black  and  the 
green  would  have  been  represented  as  brighter  than  it 
really  was,  because  of  the  deficiency  of  sensitiveness  in 
the  plate  to  red.  But,  whatever  their  imperfections,  these 
plates  are  a  very  great  improvement  on  ordinary  plates, 
when  coloured  subjects  have  to  be  dealt  with. 

It  was  demonstrated  by  Vogel  in  his  earliest  experi¬ 
ments  that  it  was  possible  to  increase  the  sensitiveness 

237 


The  Effect  of  Colour  and  its  Control 

of  a  plate  to  red  by  the  use  of  a  suitable  dye,  but  the 
difficulties  and  the  risks  that  have  to  be  incurred  in  such 
sensitising  on  a  manufacturing  scale  prevented  the  pre¬ 
paration  of  red  sensitive  plates  for  many  years,  and  those 
who  needed  them  prepared  a  few  at  a  time  for  their  own 
use.  But  eventually  red  sensitised  plates  were  put  upon 
the  market,  notably  by  Messrs.  Lumiere  and  Messrs.  Cadett, 
and  during  the  last  few  years  a  further  step  has  been 
taken  in  the  preparation  of  plates  sensitive  to  all  colours 
(panchromatic  plates)  in  as  even  a  way  as  possible. 
At  the  same  time  the  manufacture  of  colour  filters  has 
been  perfected,  and  Messrs.  Wratten  &  Wainwright  have 
specialised  in  this  matter  to  such  an  extent  that  they 
issue  a  catalogue  of  nearly  eighty  different  filters.  By 
the  joint  use  of  colour  sensitised  plates  and  colour  filters, 
the  photographer  has  at  the  present  day  almost  every¬ 
thing  that  he  can  desire  in  the  power  to  control  the  effect 
of  colour  in  his  work. 

It  is  not  unusual  to  speak  of  plates  that  have  been 
colour  sensitised,  whether  by  adding  the  dye  to  the 
emulsion  or  by  putting  the  coated  plate  into  the  dye 
solution,  as  “dyed  plates."  This  description  seems  per¬ 
haps  particularly  suitable  when  the  plate  itself  is  immersed 
in  the  solution  of  the  dye,  because  of  the  similarity  of 
the  operation  to  the  process  of  dyeing  fabrics,  but  it  is 
liable  to  lead  to  a  misunderstanding.  It  appears  to  be 
definitely  proved  that  all  dyes  are  not  serviceable  for  this 
purpose,  and  that  the  dye  only  becomes  effective  by 
forming  a  compound  with  the  sensitive  silver  salt,  and 
that  the  excess  of  the  dye  that  does  not  combine  in  this 
way  remains  soluble  and  is  advantageously  washed  away. 
Thus  it  is  the  new  compound  produced  that  gives  the 
new  sensitiveness,  and  there  is  no  need  for  the  somewhat 
fanciful  theories  that  the  dye  absorbs  the  light  of  that 
colour  to  which  sensitiveness  is  desired,  and  holding  it  as 
it  were  in  bondage  obliges  it  to  act  against  its  disposition 

238 


The  Effect  of  Colour  and  its  Control 

upon  the  silver  salt ;  nor  on  the  other  hand  that  the  light 
acts  upon  the  dye  alone  and  so  changes  it  that  the  pro¬ 
ducts  of  its  change  affect  the  silver  salt. 

It  might  be  thought  that  in  ordinary  photography, 
where  it  is  desired  to  get  a  picture  that  will  represent 
the  object  photographed  as  nearly  as  possible  as  we 
see  it,  that  the  soundest  advice  would  be  always  to  use 
panchromatic  plates  in  conjunction  with  a  light  filter 
that  will  exactly  compensate  for  their  excessive  sensitive¬ 
ness  to  blue  and  violet  and  the  ultra-violet.  In  one 
sense  such  advice  is  safe  and  good,  and  if  a  person  were 
fearful  of  being  involved  in  a  railway  accident,  it  would 
in  a  similar  sense  be  safe  and  good  advice  to  urge  them 
never  to  travel  in  a  railway  train.  But  a  policy  of  total 
abstinence  is  always  unreasonable,  except  when  it  has  to 
do  with  something  that  is  invariably  an  evil.  Other 
things  being  equal,  ordinary  plates  are  cheaper,  more 
stable,  and  require  fewer  precautions  in  their  manipula¬ 
tion  than  colour  sensitised  plates.  When  a  colour  screen 
is  used  the  exposure  must  always  be  prolonged,  and 
without  a  colour  screen  a  colour  sensitised  plate  shows 
very  little  improvement.  It  is  obviously  not  worth  while 
to  incur  the  disadvantages  associated  with  specially 
sensitised  plates  unless  there  is  something  to  be  gained 
by  their  use.  When  and  when  not  to  use  them  becomes 
therefore  a  very  practical  question.  In  a  general  sense 
colour  demands  the  most  careful  consideration  when  a 
very  obvious  colour  or  many  colours  have  to  be  dealt 
with,  as  in  pictures,  dyed  fabrics,  and  flowers.  Houses 
built  with  yellow  or  red  bricks  and  slate  roofs  appear 
to  the  eye  to  have  the  roof  much  darker  than  the 
walls ;  when  photographed  with  an  ordinary  plate  the 
probability  is  that  there  will  appear  only  a  little  differ¬ 
ence  between  the  walls  and  the  roof  or  that  the  roof 
will  appear  to  be  the  lighter.  Yellow  or  reddish  sand, 
the  greens  of  foliage  and  grass,  and  golden  or  auburn 

239 


The  Effect  of  Colour  and  its  Control 

hair,  will  come  out  too  dark' if  an  ordinary  plate  is  used. 
The  grain  of  coloured  woods  such  as  mahogany  and 
oak  is  likely  to  be  almost  lost  unless  special  precautions 
are  taken,  and  the  cracks  in  old  oil  paintings  will  often 
be  very  much  emphasised  under  the  same  conditions. 
But  on  the  other  hand,  a  black  and  white  object  such 
as  a  drawing  in  Indian  ink,  an  engraving  or  a  print,  a 
marble  or  a  bronze  statue,  a  stone  building,  and  many 
other  such  objects,  do  not  call  for  any  precautions  as 
to  colour,  and  where  the  photograph  is  made  simply  to 
record  a  position,  such  as  of  a  movable  index  or  scale, 
then  colour  may  be  absolutely  disregarded. 

There  is  one  other  case  that  is  not  generally  associated 
with  colour  but  which  really  belongs  to  the  same  class 
of  phenomena,  and  that  is  the  mistiness  that  almost 
always  affects  our  view  of  objects  at  a  distance.  This  is 
due  to  the  particles  in  the  air,  the  presence  of  which  is 
demonstrated  by  the  motes  that  may  always  be  seen 
dancing  in  a  sunbeam.  The  nature  of  the  light  that 
these  reflect  or  scatter  depends  upon  their  size.  If  they 
are  small  enough  they  scatter  or  reflect  only  the  ultra¬ 
violet  light,  and  as  this  is  invisible  the  scattering  of  it 
makes  no  difference  to  the  appearance  of  things.  But  if 
these  particles  are  larger,  then  they  affect  the  light  that 
we  can  see,  and  as  every  particle  is  illuminated  just  like 
the  things  that  we  see  the  shape  of  and  handle,  the  air 
appears  to  be  full  of  light,  and  we  call  the  appearance 
a  mist  if  we  are  in  it  and  it  is  general,  or  a  cloud  if 
we  are  not  in  it  and  its  limits  are  more  or  less  obvious. 
We  get  the  same  effect,  but  in  an  exaggerated  degree, 
when  a  passing  motor  car  stirs  up  the  dust,  but  the 
particles  in  this  case  are  larger  still  and  often  in  greater 
quantity.  Now  it  seems  that  there  are  always  small 
particles  in  the  air,  and  that  often  though  not  always 
there  are  larger  particles  also  the  effect  of  which  is 
plainly  visible.  But  the  photographic  plate  is  sensitive 

240 


The  Effect  of  Colour  and  its  Control 

to  the  ultra-violet,  and  to  this  extent  is  affected  not  only 
by  the  mistiness  that  we  can  see,  but  also  by  the  more 
universally  present  mistiness  due  to  the  ultra-violet  and 
dark  blue  light  which  is  scattered  by  the  smaller  particles 
and  not  recognisable  by  our  eyes.  Hence  it  generally 
happens,  unless  special  precautions  are  taken,  that  the 
mistiness  due  to  distance  is  very  much  exaggerated  in 
a  photograph.  The  cure  for  this  is  obvious ;  we  have 
only  to  prevent  the  ultra-violet  and  the  dark  blue  light 
which  constitute  the  mist  from  reaching  the  plate,  and 
this  is  done  by  putting  on  the  lens  a  colour  filter  that 
will  absorb  it. 

It  follows  almost  as  a  matter  of  necessity,  that  if  one 
has  the  power  to  control  the  effects  of  variously  coloured 
lights  and  so  to  bring  the  result  into  harmony  with  the 
impression  of  the  scene  that  our  eyes  receive,  that  it  is 
possible  to  overstep  the  mark  and  produce  discrepancies 
in  the  opposite  direction.  We  can  by  “  over-correction  " 
make  our  yellows,  greens,  and  reds  come  out  too  light 
and  our  blues  too  dark.  This  is  a  possibility,  though 
not  a  probability,  if  we  may  judge  by  general  results. 
But  this  very  power  is  sometimes  of  the  greatest  service. 
Engineers’  drawings  are  often  copied  by  the  ferro- 
prussiate  process,  which  gives  a  blue  image.  The  blue 
colour  of  the  print  has  no  significance ;  it  is  accepted 
merely  because  the  process  is  cheap  and  easy  to  work. 
The  blue  image  and  white  paper  give  only  a  feeble 
result  when  such  a  print  is  photographed  in  the  ordinary 
way,  but  by  causing  the  blue  to  photograph  as  if  it 
were  black  a  much  superior  result  can  be  obtained.  In 
the  study  of  cloud  forms  it  may  be  desirable  to  render 
the  blue  sky  as  if  it  were  black  and  so  to  emphasise  the 
clouds.  Biological  preparations  are  often  stained  with 
a  colouring  matter  that  will  affect  some  parts  of  them 
and  not  others,  in  order  to  bring  into  greater  promi¬ 
nence  the  important  details.  The  colour  is  used  solely 

241  Q 


The  Effect  of  Colour  and  its  Control 

for  this  purpose,  and  it  is  a  matter  of  indifference 
whether  it  is  red,  blue,  green,  purple,  or  what  it  is,  so 
long  as  it  makes  the  detail  more  clear.  It  is  easy  in 
photographing  such  a  preparation  to  intensify  the  action 
of  the  stain  and  make  the  stained  details  appear  in  the 
photograph  as  if  they  were  very  much  more  conspicuous 
than  they  really  are.  And  we  might  mention  other  cases 
where  this  possibility  of  exaggerating  the  effect  of  colour 

is  of  the  greatest  importance. 

It  is  obvious  that  it  is  possible  to  go  still  further  in 
this  direction  than  has  been  indicated,  and  take  photo¬ 
graphs  by  “light"  that  the  eye  cannot  see,  because  in 
the  spectrum  it  lies  outside  the  range  of  vision  beyond 
the  red  or  beyond  the  violet.  Professor  R.  W.  Wood 
has  taken  many  photographs  with  such  “dark  light, 
and  by  his  kind  permission  we  are  able  to  show  (facing 
this  page)  a  landscape  taken  by  means  of  infra-red 
light.  It  will  be  noticed  that  the  picture  is  quite 
different  from  what  we  see.  The  blue  sky  is  very  dark, 
the  green  foliage  and  grass  appear  as  if  they  were  very 
bright — so  bright,  indeed,  that  they  look  as  if  they  were 
covered  with  snow.  The  shadows  are  unduly  black, 
sometimes  of  a  coaly  blackness,  because  the  particles 
in  the  air,  though  they  scatter  blue  light,  are  generally 
too  small  to  affect  the  red,  much  less  the  infra-red.  If 
ultra-violet  light  is  used,  window  glass  becomes  opaque, 
Chinese  white  becomes  black,  printers’  ink  appears  as  if 
it  were  lighter  than  we  are  accustomed  to  regard  it,  and 
shadows  almost  disappear  although  the  sun  may  be 
shining,  because  the  air  is  so  misty  to  the  ultra-violet 
light  that  it  diffuses  it  as  a  fog  does.  We  see  therefore 
how  the  appearance  of  things  in  general  depends  upon 
the  sensitiveness  of  our  eyes  to  the  various  constituents 
of  light,  and  that  if  we  had  a  larger  range  of  vision,  or 
were  less  “colour  blind,”  it  would  not  be  an  unmitigated 
advantage. 


242 


R.  IV.  Wood 

Photographed  by  Infra-red  Light 


Infra-red  light  is  light  of  wave-lengths  too  long  for  the  eye  to  be  affected  by  it  to  t  e 
eye,  therefore,  it  is  not  light  at  all.  The  usual  discrepancy  that  exists  between  a  view  as 
we  see  it  and  as  it  appears  in  a  photograph  when  an  ordinary  plate  is  used  is  due  to  t  e  lg 
being  of  wave-lengths  shorter  than  those  that  chiefly  affect  the  eye.  This  difference  ere- 
fore  is  here  reversed  and  very  much  emphasised.  The  sky  is  very  dark,  the  shadows  are 
practically  black,  and  the  foliage  and  grass  are  so  light  as  to  suggest  that  they  might  have  .  i 
been  covered  with  snow. 


The  Effect  of  Colour  and  its  Control 

There  is  another  matter  closely  connected  with  this 
subject,  though  it  has  exactly  opposite  aims,  namely,  the 
selection  of  a  suitable  means  for  the  illumination  of  the 
dark-room,  the  room  in  which  sensitive  materials  may 
be  unpacked,  put  into  the  camera,  developed,  and  indeed 
manufactured.  We  want  for  this  purpose  a  light  that 
shall  be  as  bright  as  possible  to  the  eye  and  as  weak  as 
possible  in  its  action  on  the  plates  or  papers.  The 
selected  light  is  obtained  by  causing  the  source  of 
illumination  to  shine  through  a  coloured  medium.  Now 
no  colour  filter  can  be  absolute  in  its  action.  If,  for 
example,  it  stops  or  absorbs  the  blue  constituent  of  any 
ordinary  light,  and  it  is  of  such  an  intensity  that  it 
absorbs  nine-tenths  of  the  blue,  allowing  one-tenth  to 
pass,  if  a  second  layer  of  the  same  medium  is  put  against 
the  first,  it  also  will  absorb  nine-tenths  of  the  blue  that 
falls  upon  it  and  transmit  one-tenth.  If  therefore  one 
layer  allows  only  one-tenth  of  the  blue  to  pass  through 
it,  two  layers  will  allow  only  one-tenth  of  one-tenth — 
that  is,  one-hundredth — to  pass,  and  a  third  layer  would 
allow  only  one-tenth  of  this  hundredth  to  pass  through, 
and  so  on.  Thus  there  is  no  absolute  effect,  and  a  light 
that  would  produce  no  effect  upon  a  certain  plate  at  a 
certain  distance  when  exposed  to  it  for  a  certain  time, 
might  spoil  the  plate  if  the  distance  were  reduced  or 
the  time  increased  or  if  the  plate  were  a  little  more 
sensitive.  There  is  no  such  thing  as  an  absolutely  safe 
or  inactive  light.  A  dark-room  lantern  may  give  a 
practically  safe  light  with  a  candle  and  an  unsafe  light 
with  a  paraffin  lamp  or  gas  flame  ;  it  may  be  safe  at  a 
distance  of  a  yard,  but  very  unsafe  only  a  foot  away. 
It  is  therefore  essential  to  have  a  constant  light  or  one 
that  can  be  regulated,  and  this  means  that  daylight  must 
be  given  up  altogether,  except  in  the  case  of  materials 
that  are  only  slightly  sensitive.  If  a  window  were  covered 
over  with  a  coloured  medium  and  the  light  passing 

243 


The  Effect  of  Colour  and  its  Control 

through  rendered  practically  safe  in  the  winter,  it  would 
og  the  plates  in  the  summer,  it  might  be  safe  on  a  dull 
day  and  quite  unsafe  a  day  or  two  afterwards  when  the 
wither  happened  to  be  bright,  and  if  the  cobured 
medium  were  dark  enough  to  make  the  light  suitable 
in  bright  weather,  on  a  dull  day  the  amount  o  g 
would  be  so  small  as  to  be  scarcely  worth  hav‘"g- 

The  first  step  in  preparing  a  useful  light  for  the  dark 
room  is  to  remove  or  absorb  the  ultra-violet,  as  t  is 
affects  the  plate  and  is  of  no  use  whatever  to  see  by. 
The  next  step  is  to  absorb  the  blue,  as  this  has  a  con¬ 
siderable  effect  Jon  the  plate  and  is  dark  to  the  ey  . 
Blue  is  removed  by  a  yellow  screen  or  filter,  an  a  ?*  ° 
light  has  the  advantage  of  being  the  brightest  light  to 
the  eye  and  having  only  a  slight  effect  on  ordinary  plates 
For  plates  that  are  specially  sensitised  for  green 
colour  should  be  removed  as  well  as  the  blue,  and  this 
leaves  us  with  a  red  light.  With  panchromatic  plates 
the  only  safe  way  is  to  have  no  light  at  all,  or  if  ther 
is  a  feeble  light,  to  so  manage  that  the  plate  is  always 
shielded  from  it.  If  a  plate  were  equally  sensitive  to  all 
colours  and  a  light  was  necessary  for  its  manipulation, 
a  yellowish  green  light  would  be  the  best,  because  this  is 
the  brightest  to  the  eye,  but  it  would  have  to  be  reduced 
in  intensity  until  its  action  on  the  plate  was  negHgi  e 
when  exposed  to  it  for  the  required  time.  Such  a  light 
would  be  very  feeble  indeed. 


244 


CHAPTER  XV 

THE  PHOTOGRAPHY  OF  COLOUR 

Photography  is  essentially  a  monochromatic  or  u  black 
and  white”  method  of  pictorial  representation.  As  shown 
in  the  last  chapter,  it  is  affected  by  colour  because  the 
character  of  the  activity  of  light  depends  upon  its  wave¬ 
length,  as  also  does  its  colour.  Photographs  may  be  made 
of,  practically  speaking,  any  colour,  not  only  of  those 
purplish  and  reddish  browns  that  used  to  be  associated  with 
the  expression  “  photographic,”  but  of  almost  any  pigment 
or  dye,  using  such  methods  as  the  carbon  process  of  print¬ 
ing  or  photo-mechanical  processes.  But  the  reproduction 
of  colour  by  photographic  means  is  an  entirely  different 
matter,  for  to  do  this  we  must  have  several  colours  on 
the  same  photograph,  and  the  placing  of  these  colours 
must  be  effected  by  the  action  of  light.  Clearly,  too,  we 
must  not  only  get  striking  and  decided  colours,  but  all 
the  various  tones  and  tints  of  them,  and  also  black  and 
white  at  the  same  time.  It  would  be  useless  to  be  able 
to  imitate  the  tint  of  a  man's  face,  if  the  black  of  his  coat 
and  the  white  of  his  collar  could  not  be  reproduced.  And 
it  is  in  this  that  the  best  test  of  any  method  of  colour 
reproduction  depends.  It  shows  a  crudeness  of  idea  and 
a  want  of  appreciation  of  the  difficulties  of  any  method 
when  strikingly  coloured  objects  are  selected  as  tests,  a 
red-coated  soldier  rather  than  a  civilian,  or  a  bunch  of 
flowers  rather  than  a  sober  landscape.  The  exhibition  of 
the  photographs  of  such  objects  may  always  be  taken  to 
indicate  that  the  process  illustrated  is  in  its  early  stages, 

245 


The  Photography  of  Colour 

and  cannot  be  trusted  to  give  those  delicate  differences 
of  shade  that  constitute  the  true  beauty  of  all  colour. 

It  is  easy  to  superficially  describe  what  is  meant  by 
the  expression  <4a  photograph  in  colour."  In  the  produc¬ 
tion  of  a  Daguerreotype,  the  form  of  the  object  is  accurately 
rendered,  or  if  the  apparatus  is  defective  the  error  can  be 
discovered  and  definitely  expressed,  and  the  light  and 
shade  (irrespective  of  colour)  is  also  given  accurately  or 
with  deviations  from  truth  that  are  subject  to  absolute 
laws.  The  common  idea  of  a  photograph  in  natural  colours 
is  that  the  colour  of  the  object  shall  be  reproduced  with 
the  same  certainty  as  its  form  and  brightness.  We  often 
hear  people  who  know  nothing  whatever  about  it,  say  that 
they  do  not  see  why  it  should  not  be  so,  and  they  will 
prophesy  that  such  a  result  will  certainly  be  achieved. 
But  no  one  who  speaks  so  can  give  any  reason  why  it 
should  be  so,  or  offer  any  justification  for  his  prophecy. 
There  seems  to  be  but  the  feeblest  shadow  of  a  foundation 
for  the  hope  that  such  a  method  of  colour  photography 
will  ever  be  realised. 

We  may  call  this  the  direct  method  of  colour  photo¬ 
graphy,  for  it  has  been  the  dream  of  a  section  of  photo¬ 
graphic  experimentalists  for  generations,  and  a  certain 
measure  of  success  has  attended  their  efforts.  The  success 
may  be  compared  to  the  Will-o’-the-wisp  that  lures  the 
traveller  onwards  without  helping  him  home.  A  little 
more  than  a  hundred  years  ago,  Dr.  T.  J.  Seebeck  of  Jena 
noticed  in  photographing  the  spectrum  on  silver  chloride 
that  violet  light  sometimes  produced  a  violetish  reddish 
brown,  blue  sometimes  produced  a  blue,  and  red  a  rose 
or  lilac  coloured  substance.  These  results  were  recalled 
and  confirmed  when  photography  became  a  practical  art 
because  of  the  introduction  of  the  Daguerreotype,  and 
Edmund  Becquerel  in  particular  produced  layers  of  chloride 
of  silver  in  various  ways,  and  actually  succeeded  in  getting 
in  his  photographs  the  colours  of  brightly  dressed  dolls, 


The  Photography  of  Colour 

and  coloured  designs.  But  Robert  Hunt,  another  experi¬ 
mentalist,  records  how  he  once  obtained  what  he  calls  a 
very  beautiful  picture,  in  which  the  sky  was  crimson, 
stucco-fronted  houses  slaty  blue,  and  the  gieen  fields  a 
brick  red.  The  colours  obtained  by  these  methods  were 
not  permanent ;  by  carefully  preserving  the  photograph 
in  the  dark  they  might  last  for  a  few  years,  and  they  were 
destroyed  by  any  attempt  to  fix  them. 

There  is  no  doubt  that  under  certain  conditions  light, 
when  it  acts  upon  some  substances  that  it  can  change, 
tends  to  produce  a  substance  that  will  reflect  light  of  the 
same  colour  as  that  which  induced  the  change,  and  in 
1868,  Dr.  W.  Zenker  of  Berlin,  published  a  small  volume 
in  which  he  sought  for  an  explanation  of  this  tendency. 
He  pointed  out  that  this  result  is  specially  noticeable  in 
Becquerel's  method,  in  which  the  sensitive  layer  of  silver 
chloride  is  produced  on  a  silver  plate,  and  suggested  that 
the  cause  of  the  colour  was  the  production  of  metallic 
silver,  which  of  course  is  itself  not  coloured,  in  layers  in 
the  film,  something  like  the  leaves  of  a  book  with  just  a 
little  space  between  each  leaf  and  the  next,  the  width  of 
this  space  depending  on  the  colour  of  the  light  that  pro¬ 
duces  the  decomposition  of  the  silver  salt. 

In  the  first  chapter  of  this  volume,  we  endeavoured  to 
indicate  the  general  character  of  light  by  an  experiment. 
A  piece  of  cord  was  fastened  to  a  hook,  and  the  free  end 
given  a  jerk  that  caused  a  raised  portion  or  hump  to 
travel  along  the  cord.  If  the  cord  were  long  enough  the 
hump  would  be  seen  travelling  from  the  hand  to  the 
hook — but  more,  it  would  then  come  back  again,  being 
reflected  at  the  fixed  end  of  the  cord.  If  now  instead  of 
one  jerk  producing  one  hump,  a  rapid  succession  of  jerks 
is  maintained,  a  row  of  humps  will  travel  along  the  cord, 
and  every  hump  when  it  gets  to  the  fixed  end  of  the  cord 
will  come  back  again,  and  thus  there  will  a  row  of  humps 
going  in  both  directions.  The  humps  that  are  going  and 

247 


The  Photography  of  Colour 

the  humps  that  are  returning  must  affect  each  other  ;  they 
must  interfere  with  each  other,  because  the  cord  cannot 
move  in  two  directions  at  the  same  time.  The  final  effect 
of  the  succession  of  humps  travelling  along  the  cord  in 
both  directions  may  be  seen  with  a  little  care — and  it  is 
that  there  are  produced  a  number  of  humps  that  do  not 
travel  at  all.  Each  hump  keeps  moving  across  the  line 
of  the  cord,  first  above  and  then  below  if  the  jerk  given 
is  up  and  down,  so  that  the  effect  of  the  jerk  given  by 
the  hand  continues  to  pass  along  the  cord,  for  there  is 
nothing  else  that  causes  it  to  move,  but  the  humps  them¬ 
selves  do  not  travel.  Exactly  the  same  effect  is  produced 
when  light  of  any  single  wave-length  is  reflected  back 
along  its  own  path,  and  the  waves  that  are  produced 
are  called  “standing  waves”  or  “stationary  waves” 
to  distinguish  them  from  travelling  waves.  When  a 
result  such  as  this  takes  place  in  a  sensitive  film,  the 
sensitive  substance  will  be  acted  on  to  the  greatest 
extent  where  the  movement  of  the  luminiferous  ether  is 
greatest — that  is,  at  the  crests  of  the  waves — but  between 
the  waves,  at  the  nodes,  where  there  is  no  movement, 
there  will  be  no  action,  and  therefore  the  product  of  the 
change  will  be  disposed  in  layers  as  described.  Zenker 
pointed  out  that  such  layers  might  be  expected  to  reflect 
most  strongly  light  of  the  same  wave-length  as  that  which 
produced  them,  and  therefore  to  pick  out  light  of  this 
wave-length  or  colour  from  white  light  and  reflect  it. 

Professor  O.  Wiener  of  Aachen,  rather  more  than 
five-and-twenty  years  ago,  investigated  the  character  of 
the  colours  produced  by  the  direct  methods  of  colour 
photography,  and  he  found  that  the  colours  produced  by 
Seebeck’s  method  were  of  the  nature  of  pigments,  and 
did  not  give  their  colour  by  the  action  of  layers  that 
would  result  from  the  action  of  stationary  waves,  but 
that  when  a  silver  plate  was  used  as  in  Becquerel’s  method 
the  case  was  different  and  the  colour  behaved  as  if  it 

248 


The  Photography  of  Colour 

had  origin  in  such  a  manner.  Professor  Lippmann  of 
Paris,  in  1891,  perfected  this  method  of  Becquerel’s  in 
the  light  of  the  suggested  principle  upon  which  it  was 
founded,  and  so  elaborated  his  method  of  colour  photo¬ 
graphy. 

For  the  practice  of  Lippmann’s  method,  the  essentials 
are  a  film  that  contains  an  exceedingly  fine  deposit  of  the 
silver  salt,  for  a  coarse  deposit  would  clearly  interfere 
with  the  integrity  of  the  layers  that  are  so  close  and  thin  . 
the  silver  compound  must  be  sensitive  to  all  colours,  so 
that  every  colour  may  produce  its  due  effect ;  it  must  be 
transparent,  so  that  the  standing  waves  may  be  produced 
in  it  without  impeding  conditions  ;  and  there  must  be  a 
good  reflector  close  against  the  film,  to  send  the  light  back 
upon  itself.  He  prepared  a  film  on  glass  having  these 
characteristics,  and  for  a  reflector  used  mercury.  The 
plate  carrier  was  so  constructed  that  when  the  plate  was 
fixed  in  it  with  its  glass  side  towards  the  lens,  clean 
mercury  could  be  poured  in  until  the  film  side  was 
covered  with  it.  After  the  exposure  the  mercury  was 
run  out,  and  the  plate  removed  and  developed.  That 
there  really  are  these  layers  of  silver  produced  in  the 
film  by  the  standing  waves  has  been  abundantly  proved 
by  several  investigators  who  have  cut  sections  of  the  films 
and  magnified  them.  As  the  colours  shown  by  these 
photographs  depend  upon  the  distance  apart  of  these 
layers,  any  circumstance  that  affects  this  distance  must 
change  the  colours.  When  the  film  is  wetted  the  gelatine 
swells  and  the  layers  are  more  separated.  The  colours 
therefore  do  not  show  while  the  plate  is  being  de¬ 
veloped,  but  only  after  it  is  dry.  By  merely  breathing 
on  an  unprotected  film  its  expansion  is  sufficient  to  cause 
the  colours  to  change  if  they  do  not  vanish,  and  to  secure 
the  plates  against  such  changes  it  is  usual  to  cement  a 
glass  plate  on  to  the  surface  of  the  film.  To  see  the  colouis 
of  these  photographs,  the  light  must  fall  upon  them  in  a 

249 


The  Photography  of  Colour 

convenient  direction,  so  that  the  reflected  light  may  pass 
to  the  observer’s  eye.  It  would  be  useless,  for  example, 
to  hang  them  on  the  wall  as  pictures,  for  the  colours 
are  only  visible  under  the  definite  conditions  described. 
They  can  only  be  made  of  dimensions  that  are  quite 
small  compared  with  the  focal  length  of  the  lens  used, 
presumably  because  the  rays  that  fall  upon  the  margins 
of  the  plate,  if  too  oblique,  would,  after  reflection,  not 
travel  back  upon  their  original  paths.  With  all  these 
limitations  and  the  difficulties  of  the  method,  this  pro¬ 
cess  of  colour  photography  is  a  curious  experiment  or 
an  interesting  demonstration  of  the  existence  of  standing 
waves  rather  than  a  practical  process  of  colour  photo¬ 
graphy,  and  it  seems  likely  to  remain  so.  We  may  re¬ 
gard  this  as  the  final  result  of  the  attempts  to  discover  a 
direct  method  of  colour  photography. 

The  indirect  principle  of  colour  photography  comprises 
all  the  practical  methods  at  present  available,  and  it  diifers 
from  the  direct  methods  in  that  it  is  not  sought  by  photo¬ 
graphic  means  to  produce  colour  or  any  disposition  of 
silver  in  the  film  that  will  cause  colour  by  certain  arrange¬ 
ments  of  light  and  inspection,  but  merely  to  photo¬ 
graphically  regulate  the  disposition  of  pigmentary  or 
colouring  matter.  This  is  an  entirely  different  problem 
from  the  other,  and  one  that  has  been  solved  in  many 
ways.  Indeed  we  may  say  that  the  only  bar  to  perfect 
success  is  the  difficulty  of  getting  the  pigments  or  the  dyes 
of  exactly  the  right  colours  and  in  exactly  the  right  pro¬ 
portions,  and  the  difficulties  of  manipulation  that  must  be 
always  present  when  several  operations  are  necessary  to 
produce  the  desired  result. 

There  are  certain  facts  that  have  led  to  the  supposition 
that  the  ordinary  human  eye  appreciates  colours  by  means 
of  three  different  kinds  of  sensitive  points,  or  whatever 
they  may  be  called,  which  are  intermingled  on  the  retina, 
one  kind  affected  by  red  light,  one  by  green,  and  the  other 

"  250 


The  Photography  ot  Colour 

by  a  blue  that  inclines  towards  violet.  It  is  found  that  it 
is  possible  by  means  of  these  three  colours  to  imitate  any 
colour  so  closely  that  the  eye  cannot  distinguish  between 
the  original  colour  and  the  imitation.  It  is  important  to 
notice  that  this  is  not  a  question  of  mixing  pigments  or 
paints,  but  of  mixing  lights,  for  if  this  difference  is  not 
borne  in  mind  the  subject  cannot  be  understood.  A  red 
light  may  be  obtained  by  putting  a  red  glass  in  front  of 
a  lantern  and  a  green  light  by  the  use  of  a  green  glass,  as 
we  see  every  day  in  railway  signalling.  The  fundamental 
fact  is  simply  this,  that  if  we  had  three  lanterns,  one  giving 
red,  one  green,  and  one  a  violetish  blue,  and  if  these 
colours  were  properly  selected,  then  by  allowing  them  all 
to  shine  at  the  same  time  and  with  a  suitable  intensity 
upon  a  surface  as  of  white  paper,  the  surface  would  appear 
white  where  the  lights  were  superimposed.  By  altering 
the  intensity  of  any  one,  the  balance  would  be  upset,  and 
the  paper  would  appear  coloured,  and  by  suitably  regulat¬ 
ing  the  intensity  of  them,  or  by  the  use  of  any  two  of  them  or 
any  one  of  them,  all  colours  can  be  imitated.  Although  the 
eye  cannot  distinguish  between  the  original  colour  and  its 
imitation,  they  may  not  be  the  same,  and  this  can  be  proved 
by  the  separation  of  each  into  its  constituents  by  getting 
the  spectrum  of  it.  In  this  imitation  we  take  advantage 
of  the  limited  power  of  the  eye  to  discriminate  in  the 
matter  of  colour.  Whether  or  not  we  actually  see  colours 
by  the  means  of  three  different  kinds  of  sensitive  points  in 
the  eye,  does  not  affect  the  fundamental  fact  that  all 
colours  can  be  imitated  by  the  use  of  a  certain  three  ,  the 
“  three-colour5'  processes  of  colour  reproduction  therefore 
do  not  depend  upon  any  physiological  theory  as  to  colour 
vision,  but  only  on  the  experimental  facts  that  have  led  to 
such  theories. 

All  three-colour  processes  consist  in  photographing 
separately  the  redness,  the  greenness,  and  the  blueness  of 
the  object,  and  then  by  some  means  and  the  use  of  suitable 

251 


The  Photography  of  Colour 

pigments  or  dyes  getting  the  redness,  the  greenness,  and 
the  blueness  together.  There  is  then  a  photograph  of  the 
object  with  all  the  accuracy  of  an  ordinary  photograph 
and  in  its  natural  colours,  or  rather  colours  which  no 
human  eye  can  distinguish  from  them.  It  is  obvious  in 
a  moment  that  the  success  depends  upon  many  details. 
The  photograph  of  each  colour,  red,  green,  and  blue,  is 
obtained  by  photographing  the  object  through  filters  or 
coloured  screens  of  these  colours.  For  getting  the  final 
result  the  three  suitable  pigments  or  dyes  have  to  be  found, 
and  each  of  them  must  be  correctly  proportioned  to  the 
others,  for  otherwise  whichever  was  in  excess  would  give 
a  predominating  tint  over  the  whole  picture.  Absolute 
success  is  therefore  impossible  ;  it  is  only  a  matter  of  degree, 
and  as  a  matter  of  fact  three-colour  prints  may  be  met 
with  in  commerce  varying  from  those  that  are  so  perfect 
that  it  is  not  possible  by  inspection  to  find  a  fault  in  them, 
to  those  that  are  mere  travesties  of  the  process — three- 
colour  prints  certainly,  but  not  even  an  approximation  to 
the  imitation  of  the  original.  Chromo-lithography  in 
which  any  number  of  separate  colour  impressions  up  to 
twelve  or  fourteen  or  more  might  be  necessary  to  produce 
each  print,  is  now  practically  obsolete,  for  the  colours  are 
more  correctly  rendered  by  the  use  of  three  only,  provided 
that  these  are  properly  selected.  This  is  an  example  of 
the  commercial  value  of  scientific  work  that  was  carried 
out  at  first  without  any  thought  of  its  practical  applica¬ 
tions. 

All  three-colour  photography,  and  all  useful  methods  of 
colour  photography  are  included  in  this  description,  are  of 
the  general  nature  described  above.  We  have  no  wish  to 
attempt  here  to  apportion  to  each  one  of  the  innumerable 
workers  in  this  field — some  working  theoretically  and 
scientifically,  and  others  practically  and  empirically — their 
share  in  the  development  of  the  various  methods  by  which 
this  process  may  be  worked.  When  a  name  is  mentioned 

252 


The  Photography  of  Colour 

it  is  because  it  has  become  associated  with  a  specific  pro¬ 
cedure  which  it  serves  to  identify.  But  we  may  say  that 
three-colour  photography  was  known  theoretically  before 
it  was  possible,  for  practical  methods  of  photographing 
reds  and  greens  may  be  regarded  as  dating  from  1873, 
when  Vogel  discovered  the  possibility  of  sensitising  plates 
for  these  colours.  It  was  not  until  about  1890  that  colour 
photography  began  to  be  of  commercial  interest. 

There  are  two  distinct  methods  by  which  the  principles 
of  three-colour  photography  may  be  applied  to  get  a 
practical  result,  and  these  are  distinguished  as  the  “additive  ” 
and  the  “  subtractive  "  methods  respectively.  The  names 
are  self-explanatory  ;  in  an  additive  process  the  colour 
constituents  are  added,  while  in  a  subtractive  process  the 
final  result  is  obtained  by  subtracting  or  withdrawing  the 
colours  that  are  not  wanted.  It  is  necessary  to  get  a  very 
clear  conception  of  the  general  principles  involved  in  these 
two  main  divisions  of  the  various  processes  before  the 
details  of  any  one  particular  process  can  be  appreciated. 
In  our  endeavour  to  make  these  clear,  we  will  use  the 
simple  words  red,  green,  and  blue  to  indicate  that 
particular  red,  green,  and  violetish  blue  which  correspond 
to  the  three  primary  colours  from  the  physiological  aspect 
of  colour  vision. 

If  any  object  is  photographed  with  a  suitable  red  filter 
or  screen  on  the  lens  so  that  only  red  light  can  pass  into 
the  camera,  then  only  the  redness  of  the  object  will  be 
photographed,  other  colours  will  be  excluded  by  the 
colour  filter.  This  redness  will  include  not  only  the 
colour  of  those  parts  that  appear  red  but  also  of  those 
parts  of  any  colour  of  which  red  is  a  constituent,  that  is, 
any  colour  which,  when  illuminated  by  white  light,  reflects 
red  either  alone  or  mixed  with  other  colours.  A  yellow 
object  is  yellow  because  it  reflects  red  and  green  lights,  a 
purple  object  reflects  red  and  blue,  a  white  object  red, 
green,  and  blue ;  and  brown  and  other  more  complex 

253 


The  Photography  of  Colour 

shades,  as  well  as  all  colours  that  are  light  in  tint  by  being 
mixed  with  white,  reflect  some  red,  and  the  red  constituent  of 
all  these  will  be  photographed  according  to  its  intensity 
under  the  conditions  described.  The  negative  that  is 
obtained  will  have  a  deposit  of  silver  corresponding  in 
density  to  the  brightness  of  the  redness,  or  the  red  con¬ 
stituent  of  mixed  colours.  If  a  print  on  glass  is  made  from 
this  negative,  the  redness  of  the  original  will  be  represented 
by  transparent  parts,  where  there  is  less  redness  there  will 
be  a  grey  deposit ;  and  where  there  is  no  redness  there 
will  be  an  opaque  deposit,  so  that  if  a  red  glass  is  put  in 
contact  with  this  print  or  transparency  and  the  two  are 
held  up  to  the  light,  there  will  be  seen  a  true  representa¬ 
tion  of  the  redness  of  the  object.  Such  a  photograph  is  a 
record  of  the  redness.  The  greenness  and  the  blueness  of 
the  object  may  be  photographed  in  exactly  the  same  way 
by  putting  a  green  or  blue  colour  filter  in  front  of  the  lens 
so  that  only  the  required  colour  can  enter  the  camera. 
Prints  on  glass  or  transparencies  may  be  prepared  from 
these  negatives,  and  by  putting  in  contact  with  each  colour 
record  a  glass  or  film  of  its  proper  colour,  we  have  a 
complete  representation  of  the  object  in  three  parts, 
according  to  its  colour,  its  redness,  its  greenness,  and  its 
blueness.  If  the  light  that  comes  through  these  three 
photographs,  each  with  its  proper  colour  filter,  can  be 
united,  then  so  far  as  our  colours  have  been  correct  and  our 
work  free  from  error,  there  will  result  a  picture  of  the  origi¬ 
nal  with  all  its  colours  and  shades  of  colours  correctly  repre¬ 
sented.  They  must  not  be  put  one  over  the  other,  because 
each  must  have  a  full  and  equal  light,  and  if  they  were  super¬ 
posed  the  second  would  only  get  the  light  that  had  passed 
through  the  first,  and  the  third  the  small  remainder  that  had 
been  able  to  pass  through  the  other  two.  If  each  of  these 
photographs  with  its  colour  screen  attached  is  put  into 
a  separate  optical  (or  “  magic  ")  lantern,  we  can  set  three 
separate  pictures  on  the  screen — that  is,  the  redness,  the 

254 


The  Photography  of  Colour 

greenness,  and  blueness  of  the  original.  If  now  these  thiee 
pictures  are  brought  together  so  that  each  one  falls  upon 
exactly  the  same  place  of  the  screen,  then  the  three  colour 
images  are  truly  added,  and  there  results  a  vivid  picture  of 
the  original  in  its  true  colours.  This  is  an  excellent  method 
of  colour  photography  and  gives  splendid  results,  but  from 
the  apparatus  required  it  is  obviously  a  method  for  the 
lecture  theatre  only.  It  is  a  simple  “  additive”  method. 
Some  years  ago  Mr.  F.  E.  Ives  perfected  and  put  on  the 
market  his  a  chromoscope,  a  small  table  instrument  in 
which  by  an  arrangement  of  reflectors  the  light  passing 
separately  through  each  of  the  transpaiencies  with  its 
appropriate  colour  screen  was  caused  to  combine  in  the 
eye  of  the  observer,  and  when  the  whole  apparatus  was 
doubled  so  that  stereoscopic  photographs  were  taken  and 
observed,  the  result  was  as  realistic  as  it  is  possible  to 
imagine. 

The  simple  method  just  described,  simple  in  its  theory 
and  in  the  directness  of  its  method,  though  not  simple  from 
a  practical  or  commercial  point  of  view,  leaves  nothing  to 
be  desired  so  far  as  its  results  are  concerned.  L>ut  to  have 
three  separate  photographs  and  the  need  of  an  instrument 
to  unite  them  is  cumbersome  and  costly.  The  possibility 
of  doing  the  whole  thing  on  one  plate  was  understood 
more  than  forty  years  ago.  It  is  only  necessaiy  to  divide 
the  surface  of  the  plate  into  minute  patches,  too  small  to 
be  individually  recognisable  at  the  ordinary  distance  of 
vision,  and  to  colour  these  patches  red,  green,  and  blue 
alternately,  to  get  the  colour  elements  all  over  the  plate. 
Such  a  plate  will  appear  to  be  grey,  because  although  the 
light  that  passes  through  the  little  colour  patches  when 
mixed  produces  white,  the  maximum  total  of  light  that  can 
pass  through  is  one-third  of  what  would  pass  if  the  colours 
were  removed.  If  such  a  plate  is  coated  with  a  suitable 
photographic  emulsion  and  exposed  in  the  camera  so  that 
the  light  that  falls  upon  it  passes  through  the  three-colour 

255 


The  Photography  of  Colour 

screen  before  coming  to  the  emulsion,  then  if  the  object  is, 
for  example,  red,  the  red  light  will  pass  through  the  little 
red  patches  and  produce,  on  development,  an  opaque 
deposit  of  silver  behind  each  red  patch.  The  red  light 
cannot  pass  through  the  green  and  blue  patches,  and  there¬ 
fore  produces  no  effect  behind  them.  If  the  developed 
plate  were  fixed  in  the  ordinary  way,  we  should  get  exactly 
the  reverse  of  what  we  want,  so  far  as  colour  is  concerned, 
for  the  red  that  we  want  would  be  stopped  by  the  opaque 
deposit  behind  it,  and  the  green  and  blue  that  we  do  not 
want  would  be  left  with  nothing  to  hinder  the  light 
passing  through  them.  But  if  instead  of  dissolving  away  the 
unchanged  silver  salt  as  in  ordinary  fixing,  we  dissolve  away 
the  image  of  metallic  silver,  which  can  be  easily  done  by 
suitable  means,  and  the  silver  salt  that  remains  is  then 
exposed  to  light  and  developed  so  as  to  get  an  opaque 
deposit,  then  on  holding  up  the  plate  to  the  light  we  shall 
see  red  only,  wherever  red  light  fell  upon  the  plate  when  it 
was  exposed  in  the  camera.  In  an  exactly  similar  way  the 
other  colours,  whether  simple  or  mixed,  will  be  repro¬ 
duced.  By  this  dissolving  away  of  the  silver  image  instead 
of  the  ordinary  fixing,  what  would  have  been  a  negative  is 
made  into  a  positive,  which,  as  was  explained  in  detail  in 
the  previous  case,  is  what  is  required.  Such  a  process  is 
called  a  “  screen-plate  process  ”  to  indicate  that  the  screen 
and  the  plate  are  in  one. 

The  difficulty  of  working  out  a  practical  process  on 
the  lines  just  described  was  to  get  these  minute  patches 
of  the  three  colours,  and  several  of  the  methods  that  were 
successful  experimentally,  failed  when  attempts  were  made 
to  work  them  on  a  commercial  scale.  The  early  experi¬ 
menters  put  the  colours  on  in  alternating  lines,  ruling 
them  in  coloured  inks  or  paints.  But  it  was  impossible 
to  rule  lines  of  the  fineness  necessary  with  no  spaces 

256 


By  permission  of  the  “  Psychcological  Review." 


By  permission  of  the  “ Psychological  Review  ” 

Optical  Illusions 

These  illustrations  show  that  the  unassisted  eye  may  be  deceived,  even 
in  spite  of  a  careful  and  prolonged  inspection  of  a  design.  It  is  difficult 
to  believe  that  the  letters  LIFE  are  upright,  and  that  the  lower  illustra¬ 
tion  consists  of  concentric  circles. 


The  Photography  of  Colour 

between  them  and  no  overlapping.  The  first  successful 
method  was  perfected  by  Messrs.  Lumiere  of  Lyons  when 
they  issued  their  autochrome  plates  in  1907.  Most 
starches  consist  of  minute  rounded  granules,  each  com¬ 
plete  in  itself,  and  which  when  heated  in  water  will  burst. 
In  the  preparation  of  autochrome  plates  Messrs.  Lumiere 
take  starch  of  a  suitably  sized  granule  and  dye  it  in 
separate  batches  red,  green,  and  blue,  and  dry  it  with 
suitable  care  that  the  granules  may  remain  perfect.  These 
dyed  granules  are  then  mixed  in  such  proportions  that 
neither  of  the  colours  predominates,  and  the  mixture  is 
spread  upon  a  glass  plate  that  has  a  sticky  coat  of  varnish 
upon  it.  The  granules  adhere,  and  when  the  plate  is 
completely  covered  and  the  excess  removed  it  is  pressed 
to  flatten  out  the  round  granules  and  fill  up  the  minute 
spaces  that  may  be  left  between  them.  The  filling  up 
of  the  interspaces  may  be  done  more  completely  by  the 
application  of  a  fine  black  pigment  which  will  adhere 
only  where  the  sticky  coat  of  varnish  is  uncovered.  A 
layer  of  a  waterproof  varnish  is  applied,  and  on  this  the 
panchromatic  photographic  emulsion  is  coated.  One 
disadvantage  of  the  use  of  starch  granules  is  that  they 
are  not  very  transparent,  and  so  cause  a  further  loss 
of  the  light,  which  has  already,  by  the  colours  themselves, 
been  reduced  to  one-third  or  less.  Since  the  autochrome 
plates  were  introduced,  three  or  four  other  firms  have 
started  the  manufacture  of  similar  colour  plates,  but 
using  coloured  inks  or  dyed  gelatine  instead  of  starch 
granules,  and  arranging  the  colours  in  regular  order 
instead  of  at  random.  The  “Thames”  plates  had  two 
colours  as  round  dots  and  the  third  filling  up  the  inter¬ 
spaces.  The  “  omnicolore  ”  plates  have  alternating  narrow 
and  rather  wider  lines.  The  narrow  lines  are  of  one 
colour,  and  the  wider  lines  consist  of  alternating  squares 

257  R 


The  Photography  of  Colour 

of  the  other  two  colours.  Clearly  many  other  arrange¬ 
ments  of  the  colours  are  possible  ;  it  is  only  a  matter  of 
convenience. 

By  the  use  of  these  screen  colour  plates  many  excellent 
results  have  been  obtained,  but  the  getting  of  colours  that 
match  those  of  the  original  is  not  by  any  means  ensured 
by  blindly  following  the  instructions  supplied  for  their  use. 
There  is  one  notable  fact  in  connection  with  them,  namely, 
that  the  proportional  intensities  of  the  three  colours  is 
fixed  by  the  maker.  This  is  an  advantage  to  the  unskilful 
worker,  because  he  has  so  much  less  opportunity  for 
making  mistakes.  But  the  opportunity  for  error  is  not 
eliminated,  though  the  adjustment  of  the  colours  in  the 
first  case  is  done  by  those  who  are  accustomed  to  the 
work,  and  are  therefore  likely  to  be  more  successful  than 
any  one  of  little  experience. 

The  production  of  transparencies  by  the  subtractive 
principle  in  colour  photography,  though  radically  different 
from  their  production  by  the  additive  principle,  is  in  a 
general  sense  very  similar  to  it.  The  redness,  greenness, 
and  blueness  are  photographed  exactly  as  before,  and  from 
each  negative  a  print  is  obtained,  the  aim  being  that  the 
three  prints  when  suitably  coloured  shall  be  superposed 
so  as  to  make  one  single  print  or  transparency.  This 
method  was  suggested  long  ago,  and  the  simplest  way 
of  carrying  it  into  effect  was  applied  by  Mr.  Ives,  but  it 
is  generally  associated  with  the  name  of  Mr.  Sanger 
Shepherd,  as  it  is  his  firm  that  has  put  it  on  a  sound 
practical  and  commercial  basis,  and  that  supplies  the 
necessities  for  it.  It  is  in  the  colour  of  the  three  prints 
that  the  difference  lies  between  the  additive  and  the  sub¬ 
tractive  methods  of  colour  photography.  When  the 
coloured  lights  are  added,  the  red,  green,  and  blue  together 
give  white  upon  the  screen  ;  but  if  red,  green,  and  bleu 


The  Photography  of  Colour 

films  were  superposed  and  held  up  to  the  light,  the  com¬ 
bined  film  would  appear  black,  for  each  would  allow  only 
one  colour  to  pass  through  it,  and  this  would  be  stopped 
by  the  other  two.  We  therefore  cannot  use  these  colours 
when  the  films  are  to  be  superposed.  In  the  additive 
method  the  position  and  intensity  of  the  colour  in  each 
case  was  regulated  by  a  black  image  that  excluded  it 
where  it  was  not  wanted.  This  is  clearly  impossible  when 
the  films  are  superposed,  because  the  second  and  third 
colours  are  wanted  in  the  very  places  where  the  first  is 
stopped  by  the  opaque  image. 

These  apparent  difficulties  are  surmounted  by  using  the 
complementary  colours  in  the  complementary  positions. 
That  is,  instead  of  putting  where  we  want  red,  a  red  dye 
that  will  allow  only  red  to  pass  through  it,  we  put  every¬ 
where  else  a  dye  that  will  allow  the  other  two  colours  to 
pass.  As  the  other  two  colours  are  green  and  blue,  the 
colour  used  for  the  red  plate  is  a  greenish  blue.  The 
colour  used  for  the  green  plate  is  a  pink,  which  allows  the 
red  and  blue  to  pass  but  not  the  green,  and  this  is  put 
everywhere  except  where  green  is  wanted.  And  yellow, 
which  allows  red  and  green  to  pass,  is  used  for  the  green 
plate.  Suppose  that  there  is  a  pure  red  in  the  object, 
the  film  that  corresponds  to  the  redness  of  the  object  is 
here  colourless,  and  therefore  allows  red,  green,  and  blue 
to  pass  through  it.  But  in  this  very  place  the  film  that 
corresponds  to  the  greenness  of  the  object,  as  it  allows  only 
the  red  and  blue  to  pass,  stops  the  green,  and  the  film 
that  corresponds  to  the  blueness  of  the  object,  allows 
only  the  red  and  green  to  pass  and  stops  the  blue,  so  that 
the  sum  of  the  effects  of  the  three  films  in  this  particular 
part  is  that  green  and  blue  are  absorbed  and  red  alone  gets 
through,  the  colour  that  corresponds  with  the  coloui  of 
the  object.  This  will  perhaps  be  better  understood  from 

259 


I 


The  Photography  of  Colour 

the  adjoining  table.  The  blank  spaces  where  the  coloui 
of  the  object  and  the  record  correspond  show  that  there 
is  no  colour — the  film  is  colourless  and  transparent.  It 
is  now  clear  why  this  method  is  called  the  subtractive 


Red  Object. 

Green  Object. 

Blue  Object. 

Dye  Used. 

Red  record  . 

Green  record 

Blue  record  . 

Absorbs  green 
Absorbs  blue 

Absorbs  red 

Absorbs  blue 

Absorbs  red 
Absorbs  green 

Bluish  green 
Pink 
Yellow 

Remaining  Colour 

Red 

Green 

Blue 

... 

method,  for  we  get  the  colours  that  we  want  by  removing 
one  at  a  time  those  that  we  do  not  want. 

In  the  Sanger  Shepherd  method  a  print  is  made  from 
the  red  record  as  if  an  ordinary  lantern  slide  were  being 
made,  and  the  image  produced  by  development  is  changed 
into  a  greenish  blue  image  by  toning  it.  Prints  from  the 
green  and  blue  records  are  made  in  gelatine  sensitised  with 
potassium  bichromate  and  supported  on  a  thin  celluloid 
film.  These  are  dyed  the  proper  colours  and  cemented  on 
to  the  first  with  Canada  balsam.  The  three  films  may  be 
put  together  tentatively,  and  if  either  colour  predominates 
or  is  deficient,  the  faulty  film  may  be  dyed  more  deeply, 
or  some  of  the  dye  may  be  washed  out,  before  the  final 
cementing  and  binding  together  of  the  complete  picture. 
Colour  transparencies  made  in  this  way  are  more  brilliant 
than  is  possible  by  any  single  plate  (or  screen  plate)  process 
because  the  whole  surface  of  the  picture  is  available  for 
each  colour. 

The  “  pinatype "  process  was  introduced  in  1895  by 
Messrs.  Meister,  Lucius,  and  Bruning.  In  this  process 
transparencies  are  made  from  the  ordinary  three  negatives, 
and  plates  coated  with  bichromated  gelatine  are  exposed 
under  these,  washed,  and  put  into  the  appropriate  dye 

260 


The  Photography  of  Colour 

solution.  Where  the  light  has  acted  on  the  gelatine,  it  has 
been  rendered  less  able  to  absorb  water,  so  that  the  dye  is 
absorbed  to  the  greatest  extent  where  the  light  has  had 
the  least  action.  Each  of  these  dyed  plates  is  then  brought 
in  turn  into  close  contact  with  a  sheet  of  glass  or  paper 
that  has  been  coated  with  gelatine,  and  the  dye  passes 
partly  into  this  gelatine  film  to  form  the  picture.  The 
final  result  therefore  consists  of  only  one  him  of  gelatine 
into  which  the  three  dyes  have  been  absorbed,  and  differs 
from  the  results  produced  by  the  Sanger  Shepherd  method, 
in  which  the  films  are  separately  dyed  and  then  cemented 
together.  Another  method  consists  in  the  preparation  of 
three  carbon  prints  of  the  necessary  colours  and  tians- 
ferring  these,  one  over  the  other,  on  to  the  same  support. 

But  the  possibilities  of  this  method  of  colour  photo¬ 
graphy  are  not  exhausted  by  these  more  purely  photo¬ 
graphic  methods.  Any  photomechanical  method  is 
available  if  three  suitably  coloured  inks  are  to  be  had, 
two  at  least  of  which  must  be  transparent  to  allow  the 
other  colours  to  show  through  them.  Excellent  work  is 
done  by  collotype,  photolithography,  photogravure,  and 
typographic  blocks,  three  plates  or  blocks  being  prepared 
in  each  case,  and  superposed  impressions  taken  fiom  them 
in  the  three  necessary  colours.  The  old  method  of  chromo¬ 
lithography,  in  which  the  colours  were  chosen  empirically 
and  the  number  of  impressions  necessary  for  each  print 
might  amount  to  a  dozen  or  more,  is  now  practically 
superseded  by  the  three-colour  process.  The  quality  of 
three-colour  prints  varies  enormously  according  to  the 
skill  exercised  in  their  production.  Pictures  may  be  copied 
so  perfectly  that  at  a  distance  of  a  yard  or  so  the  original 
and  the  copy  cannot  be  distinguished,  or  hideous  things 
may  be  produced  in  which  trees  are  pink  and  grass  is 
yellow. 

There  is  another  section  of  three-colour  methods,  in 
which  the  colours  are  not  of  a  pigmentary  or  dye-like 

261 


The  Photography  of  Colour 

character,  but  are  produced  by  optical  means,  interference, 
or  dispersion,  but  this  we  refer  to  only  to  show  the  uni¬ 
versality  of  the  application  of  the  three-colour  piinciple, 
for  pictures  so  produced  are  little  more  than  scientific 
curiosities.  We  may  perhaps  qualify  this  statement  with 
regard  to  a  method,  different  from  any  previously  described, 
which  serves  for  copying  coloured  transparencies,  but  is  far 
too  slow  for  general  work  with  a  camera.  T.  he  prepared 
paper  is  coated  with  a  film  in  which  are  three  suitable 
colours,  so  that  the  coating  is  black.  The  dyes  chosen 
are  such  that  are  bleached  by  exposure  to  light,  and  their 
instability  is  enhanced  by  incorporating  a  sensitiser 
with  the  film.  When  such  a  prepared  sheet  is  exposed 
to  light  under  a  coloured  transparency,  the  colours  of  the 
transparency  are  reproduced  more  or  less  on  the  print 
because  the  light  bleaches  the  colours  that  are  not  wanted. 
The  method  depends  upon  the  fact  that  light  can  affect  a 
substance  only  when  it  is  absorbed,  and  therefore  when 
a  mixture  of  unstable  coloured  substances  is  exposed  to  a 
coloured  light,  there  is  always  a  tendency  for  those  sub¬ 
stances  that  are  of  the  same  colour  as  the  light  to  survive 
the  longest,  because  they  reflect  more  of  that  light  than 
of  the  others.  Whatever  one  may  think  as  to  the  possi¬ 
bility  of  a  process  of  this  kind  becoming  of  general 
applicability,  it  must  be  admitted  that  the  “  Uto  ”  paper 
put  upon  the  market  a  few  years  ago  by  Dr.  J.  H.  Smith 
was  surprisingly  successful,  and  since  then  Dr.  Smith  has 
made  further  improvements  in  the  paper  that  he  has 
recently  introduced. 

Thus  we  see  from  this  rapid  review  of  the  methods  of 
colour  photography,  that  in  the  processes  that  are  com¬ 
mercially  available,  the  aim  is  not  to  reproduce  colour 
but  to  imitate  it.  The  perfection  of  the  imitation  depends 
not  only  upon  the  characteristics  of  the  method  but  upon 
the  skill  of  the  worker,  and  the  colour  shown  in  the 
result  is  not  impersonal  evidence  of  the  colour  of  the 

262 


The  Photography  of  Colour 

original.  But  with  due  care,  there  is  less  liability  to 
variation  than  if  the  whole  of  the  colouring  were  done  by 
hand,  and  the  variation  in  any  given  series  of  prints  is 
more  likely  to  be  uniform,  and  in  any  one  example  should 
be  uniform  over  the  whole  of  it. 


CHAPTER  XVI 

TRUTH  AND  ERROR  IN  PHOTOGRAPHY 


In  the  preceding  chapters  we  have  considered  the  nature 
of  light  and  how  it  is  controlled  for  photographic  purposes. 
We  have  seen  something  of  the  development  of  photo¬ 
graphy  and  of  its  possibilities.  It  will  perhaps  be  of 
advantage  to  pause  before  considering  some  of  its  special 
applications  to  look  at  photography  from  a  more  general 
point  of  view.  Essayists  have  sometimes  set  themselves 
to  catalogue  the  uses  of  photography.  That  may  have 
been  worth  doing  a  generation  ago,  but  to-day  one  might 
as  well  attempt  to  set  down  all  the  purposes  that  can  be 
served  by  writing  or  printing.  We  can  photograph  every¬ 
thing  that  we  can  see,  and  many  things  that  we  cannot  see 
or  ever  hope  to  see.  As  a  method  of  recording,  therefore, 
photography  surpasses  the  observing  power  of  human 
vision  in  the  universality  of  its  applicability,  and  it  sur¬ 
passes  it  also  in  its  truthfulness,  because  it  is  free  from 
personality  or  bias. 

Sometimes  we  are  met  with  the  question  :  Can  photo¬ 
graphy  lie  ?  And  we  hear  at  different  times  an  emphatic 
yes,  and  an  equally  emphatic  no,  given  in  reply.  Of 
course  photography  cannot  lie,  because  all  photographic 
results  are  due  to  the  effects  of  the  unchanging  laws  of 
nature.  But  a  person  may  be  misled  by  a  photograph, 
just  as  he  may  be  misled  by  a  document,  by  reason  of  his 
own  inability  to  understand  it.  And  a  photographer  may 
set  himself  to  produce  a  deceptive  photograph,  seeking  to 
gain  a  dishonest  advantage  for  himself  or  his  customer. 


Truth  and  Error  in  Photography 

But  a  photograph,  so  long  as  it  is  a  photograph  and  is  not 
sophisticated  by  hand-painting  or  other  such  processes, 
must  always  have  a  definite  relationship  to  the  thing 
photographed,  according  to  the  conditions  under  which 
the  photograph  was  produced.  In  critical  cases  it  is 
no  more  than  fair  that  photographs  should  be  inter¬ 
preted,  when  disputes  or  doubts  arise,  by  those  who  have 
studied  the  subject,  just  as  a  legal  document  is  taken  to  a 
court  of  justice  when  a  final  opinion  is  required  as  to  its 
meaning. 

The  honest  photographer,  so  far  as  he  is  really  honest, 
endeavours  to  be  transparently  honest,  and  to  produce 
work  that  not  only  is  right,  but  that  appears  to  be  right. 
This  is  the  aim  of  all  honest  persons  who  have  to  record 
facts.  But  it  is  astonishing  how  liable  a  man  who  has 
the  very  best  intentions  is  to  deceive  through  want  of 
care.  And  on  the  other  hand,  many  persons  would  be 
surprised  if  they  knew  how  easily  they  can  be  deceived. 
These  are  commonplace  facts  that  concern  every  phase 
and  detail  of  life,  including  photography.  If  the  casual 
observer  looks  at  the  illustrations  facing  page  256,  it  will 
seem  to  him  absurd  to  suggest  that  the  letters  LIFE 
are  upright  and  that  there  are  concentric  circles  in  the 
other  figure,  because  the  letters  appear  to  be  inclined 
to  each  other,  and  the  circles  appear  as  if  they  formed 
a  spiral.  But  as  the  letters  and  the  circles  are  as  stated, 
the  real  fault  is  in  the  observer,  who  does  not  see  the 
facts  as  they  are.  Nevertheless,  a  photographer  who 
produced  a  picture  that  was  strictly  correct,  if  it  pro¬ 
duced  a  false  impression  in  the  minds  of  a  large  pro¬ 
portion  of  those  who  saw  it,  would  merit  blame,  because 
it  is  the  duty  of  one  who  records  facts  to  do  his  best 
to  demonstrate  them  also.  Indeed,  in  some  matters  one 
might  be  inclined  to  allow  a  little  license  in  fact,  if  the 
impression  produced  is  correct,  but  in  photography  this 
is  not  permissible.  The  facts  should  be  correctly  re- 

265 


Truth  and  Error  in  Photography 

corded,  fundamentally  and  absolutely,  and  a  skilful  and 
honest  photographer  will  see  that  his  record  is  not  likely 
to  produce  any  false  idea. 

A  photograph  always  records  a  fact.  A  thought  or 
an  idea  may  be  expressed  in  various  ways,  but  never  by 
photography.  We  cannot  photograph  unless  there  is 
a  thing  to  be  photographed,  and  all  that  a  photograph 
can  do  is  to  represent  that  thing.  A  person  may  be 
dressed  up  to  represent  Hamlet,  and  he  may  be  photo¬ 
graphed.  The  photograph  is  not  of  Hamlet,  but  only 
of  the  person  dressed  to  represent  him,  and  whatever 
merit  there  may  be  in  the  picture  as  reminding  one  of 
Hamlet,  has  to  do  with  the  person  and  the  dressing,  and 
not  the  photography. 

That  a  photograph  always  represents  facts  and  nothing 
else  but  facts,  is  its  chief  characteristic.  An  artist  can  paint 
a  picture  of  a  milkman  filling  up  his  cans  at  the  pump, 
and  his  picture  may  be  a  pure  fancy,  it  cannot  be  direct 
evidence  that  any  milkman  ever  did  such  a  thing.  But 
if  we  have  a  photograph  of  a  milkman  at  the  pump, 
there  must  have  been  a  pump  or  an  imitation  of  one, 
and  there  must  have  been  a  person  dressed  as  a  milkman, 
and  whatever  he  is  shown  by  the  photograph  to  be  doing, 
he  must  have  been  doing  when  the  photograph  was  taken. 
In  this  sense  photography  cannot  lie.  But  there  might 
be  an  inclination  on  the  part  of  some  to  consider  that 
the  photograph  was  evidence  of  facts  which  it  has  not 
recorded,  as  it  is  often  difficult  for  an  expert  and  trained 
person  to  distinguish  between  observation  and  inference, 
and  those  who  have  never  troubled  themselves  about 
this  matter  generally  mix  their  observations  and  inferences 
in  hopeless  confusion.  In  this  particular  example  the 
connection  of  milk  with  the  episode  in  any  way  is  a 
matter  of  inference,  the  photograph  cannot  show  it,  and 
if  the  contents  of  the  vessels  were  shown  in  the  picture 
there  could  hardly  be  evidence  that  it  was  milk  and  not 


Truth  and  Error  in  Photography 

whitewash.  That  the  man  is  a  milkman  is  simply  a 
matter  of  inference  from  his  appearance  and  his  surround¬ 
ings.  If  a  plasterer  were  using  a  can  similar  in  shape 
to  a  milk  can,  and  was  getting  water  for  the  purposes 
of  his  trade,  the  photograph  would  be  much  the  same 
as  if  a  milkman  were  fraudulently  increasing  the  bulk 
of  his  milk.  And  obviously  this  matter  might  be  pursued 
further.  The  photograph  is  right  enough,  the  difficulty 
lies  in  the  interpretation  of  it. 

It  is  possible  by  photography  to  misrepresent  things, 
and  we  will  take  a  few  very  common  examples  in  order 
to  get  an  idea  of  the  general  character  of  such  misre¬ 
presentations.  But  it  must  be  borne  in  mind  that  every 
photograph,  whoever  takes  it  and  whether  by  means  of 
a  five-shilling  or  a  fifty-guinea  apparatus,  is  the  necessary 
result  of  the  procedure,  and  in  this  sense  is  a  mechanical 
production.  Its  errors,  if  any,  will  not  be  random  errors 
such  as  a  drawing  done  by  hand  is  liable  to,  but  they 
will  be  regular  and  strictly  definable,  if  the  conditions 
are  known  under  which  the  photograph  was  made. 

It  is  a  rule  in  plane  perspective  that  vertical  lines  in 
the  object  shall  be  shown  as  parallel  in  the  picture,  and 
this  is  secured  by  having  the  plate  vertical  during  its 
exposure  in  the  camera.  But  if  the  plate  is  not  vertical 
— if,  for  example,  the  photographer  finds  that  the  building 
is  too  lofty  to  get  its  image  on  the  plate  as  it  is,  and  he 
tips  up  the  camera,  with  the  result  that  the  plate  leans 
backwards,  then  the  scale  of  the  image,  as  compared  with 
the  object,  will  decrease  regularly  from  the  upper  to  the 
lower  part  of  the  plate.  The  image  of  the  upper  part 
of  the  building  falls  on  the  lower  part  of  the  plate,  so 
that  the  result  in  the  final  photograph  will  be  that  the 
top  of  the  building  will  be  represented  on  a  smaller  scale 
than  the  lower  part,  and  so  the  vertical  lines  will  be 
represented  as  converging  upwards.  This  gradation  of 
scale  and  the  consequent  convergence  is  unconventional 

267 


Truth  and  Error  in  Photography 

and  liable  to  deceive,  and  therefore  wrong,  although  it 
is  the  only  result  possible  under  the  given  conditions. 

In  the  case  just  considered  it  is  possible  to  state  ab¬ 
solutely  that  the  vertical  lines  in  the  object  should  be 
parallel  in  the  picture  and  that  any  departure  from  parallel¬ 
ism  is  an  error.  We  may  compare  this  to  stealing,  be¬ 
cause  of  it  we  may  also  say  absolutely  that  it  is  wrong 
to  take  what  belongs  to  another.  But  there  are  matters  ; 
in  which  the  error  lies  only  in  exaggeration.  Total  abstin¬ 
ence  from  food  and  gluttony  are  both  wrong  :  virtue  here 
lies  in  the  intermediate  course,  which  cannot  be  absolutely 
defined.  No  one  can  say  after  his  meal  that  half  an 
ounce  more  would  be  half  an  ounce  too  much,  or  1 1a 
to  have  eaten  half  an  ounce  less  than  he  has  eaten  would 
have  been  detrimental  to  him.  There  is  a  wide  scope 
for  discretion  ;  but  we  are  all  agreed  that  to  injure  ones 
constitution  by  either  abstinence  or  excess  is  sinful  and 
may  be  criminal.  In  pictorial  representation  a  distant 
object  is  shown  on  a  smaller  scale  than  a  nearer  objec  , 
and  it  must  be  so,  because  as  any  object  approaches  the  > 
eye  it  hides  an  increasing  extent  of  the  view  beyond  it. 
The  gradation  of  scale  according  to  the  distance  of  the 
object  is  natural  and  desirable,  but  it  may  be  carried  to  , 
excess  until  it  becomes  a  fault.  It  is  always  well  to  avoid  j 
having  too  near  a  foreground  when  the  distance  is  in¬ 
cluded  in  the  picture,  because  when  the  difference  of 
scale  is  too  great  a  pebble  in  the  foreground  may  be  J 
made  to  appear  like  a  rock,  and  a  tree  just  above  it  in 
the  distance  may  be  dwarfed  to  a  bush.  It  may  be  sug-;j 
gested  that  a  pebble  remains  a  pebble  and  does  not  seem  j 
to  «row  any  larger  when  picked  up  and  examined  morei 
closely,  and  that  is  quite  true.  If  you  will  hold  up  one 
hand  at  about  ten  inches  from  your  eye  and  the  other  u 
at  twice  the  distance,  you  will  not  notice  any  difference  I 
in  their  apparent  sizes;  but  now  shut  one  eye  and  while! 
keeping  both  hands  at  the  distances  named  bring  them 


Truth  and  Error  in  Photography 

into  line,  and  you  will  find  the  nearer  hand  is  large  enough 
to  cover  four  hands  at  the  greater  distance — that  is,  the 
nearer  hand  appears  to  be  twice  as  long  and  twice  as 
broad  as  the  more  distant.  It  is  because  we  know  that 
our  hands  are  about  the  same  size  that  they  appear  to 
be  so,  irrespective  of  varying  distance,  unless  they  are 
compared  by  bringing  them  into  the  same  line.  That 
this  is  a  matter  of  experience,  and  not  a  matter  of  estimat¬ 
ing  size,  can  be  shown  by  attempting  to  compare  objects 
of  which  you  have  no  experience.  What  round  thing 
will  be  sufficiently  large  when  held  at  arm’s  length  to 
cover  the  disc  of  the  sun  or  the  moon  ?  Those  who  have 
not  tried  the  experiment  will  make  all  sorts  of  answers 
to  this  question,  starting  with  a  dinner-plate  perhaps,  while 
others  may  suggest  a  half-crown.  As  a  matter  of  fact, 
a  threepenny-piece  is  far  larger  than  necessary.  Or  select 
a  word  in  a  printed  page,  and  guess  what  coin  will  just 
cover  it.  Such  exercises  as  these  will  show  that  our 
ideas  of  size  fail  us  when  we  compare  things  that  we  have 
not  been  in  the  habit  of  associating  together. 

Thus  we  have  no  guide  to  the  difference  in  scale 
permissible  in  the  same  picture,  except  the  appearance 
of  the  picture  as  a  whole.  If  any  part  of  the  object 
photographed  is  unduly  near  the  camera,  it  may  give 
so  large  an  image  compared  with  the  other  and  more 
distant  parts  of  the  object,  that  the  result  appears  grotesque 
if  we  know  the  thing  that  is  represented.  If  we  do  not 
know  the  object  itself,  we  shall  probably  be  deceived,  and 
it  is  possible  for  a  photographer  to  fraudulently  deceive 
in  this  way.  By  getting  close  to  a  small  garden,  he  may 
make  it  appear  to  be  almost  a  park,  and  a  pond  may  be 
represented  as  a  lake,  and  any  one  who  has  only  the 
photograph  to  guide  him,  may  be  deceived.  In  this 
sense  photography  can  lie.  In  all  such  cases  the  per¬ 
spective  will  be  correct  if  proper  care  is  taken,  though 
it  is  what  is  called  “violent”;  and  if  the  picture  is  put 

269 


Truth  and  Error  in  Photography 

in  an  apparatus  that  obliges  one  to  look  at  it  with  one 
eye  from  a  certain  correct  position,  the  suggestion  of 
exaggeration  will  disappear,  and  the  representation  of  the 
object  will  be  true  to  it.  But  without  this  limitation  in 
the  manner  of  viewing  it,  such  a  photograph  must  be 
condemned. 

There  is  another  peculiarity  in  plane  perspective, 
or  the  picture  as  given  by  a  lens  on  a  vertical  plate, 
due  to  the  flatness  of  the  plate.  Suppose  that  a  row  of 
round  or  similarly  shaped  objects,  such  as  a  row  of 
pillars,  persons,  statues,  or  balls,  is  photographed,  and 
suppose  that  all  the  pillars,  persons,  statues,  or  balls 
are  of  the  same  diameter,  they  will  not  be  represented 
as  of  the  same  diameter  in  the  photograph.  This  is  a 
simple  result  of  the  conditions,  and  to  a  certain  extent  is 
necessary  for  the  correct  representation  of  the  things, 
and,  as  in  the  previous  case,  the  fault  lies  in  the  exaggera¬ 
tion.  It  is  quite  easy  to  understand  the  reason  for  this 
difference  by  an  examination  of  the  lower  figure  facing 
this  page.  The  round  objects  are  shaded,  and  the 
marginal  rays  from  each  are  represented  by  lines  which  | 
indicate  the  extent  on  the  plate  of  the  image  of  each 
object.  The  increased  size  of  the  marginal  images  is  ! 
due  to  the  obliquity  of  the  light  rays  that  fall  upon  the  j 
plate,  and  means  therefore  a  radial  extension  of  the  j 
image  in  every  direction  from  the  plate  centre, 
photographs  of  public  functions  that  take  place  in  rooms, 
the  restrictions  as  to  space  often  oblige  the  photographer 
to  have  some  of  the  people  quite  near  to  the  camera 
and  to  include  a  wide  angle  of  view.  Those  persons  that 
are  represented  near  the  margin  of  the  plate  will  appear 
perceptibly  stouter  than  they  really  are,  or  if  they  are 
sitting  down,  their  heads  may  be  seen  drawn  out  into 
quite  an  unnatural  shape.  The  author  has  such  a  print 
in  which  one  gentleman’s  head,  the  nearest  to  the  camera 
and  therefore  the  largest  in  the  picture,  comes  unfor- 

270 


Reticulation 

Example  of  the  reticulation  of  a  gelatine  film.  This  was  obtained  accidentally,  and 
is  very  coarse.  It  shows  clearly  the  general  character  of  this  phenomenon. 


Distortion  at  the  edges  of  the  Plate  due  to  its  Flatness 

As  the  rays  that  form  the  image  fall  obliquely  on  the  plate  towards  its  edges,  the 
image  is  spread  out  in  a  radial  direction.  If  the  plate  were  curved,  as  shown  Dy 
the  dotted  line,  the  image  of  the  more  distant  object  would  be  narrower  than  that  ot 
the  nearer  object. 


Truth  and  Error  in  Photography 

tunately  near  to  the  corner  of  the  plate,  and  his  head  is 
extended  to  such  a  great  degree  and  askew  that  one 
dare  not  reproduce  it.  But  the  two  illustrations  that  face 
page  206  show  the  effect  on  inanimate  objects  that 
cannot  object  to  their  misrepresentation.  The  one  is  a 
terrestrial  globe,  and  this  fact  may  be  accepted  as  sufficient 
evidence  that  it  is  for  practical  purposes  a  sphere,  but 
by  photographing  it  so  that  its  image  falls  near  the  edge 
of  the  plate,  its  image  is  drawn  out  into  an  egg-shaped 
figure,  and  the  other  is  a  cylinder  that  is  higher  than 
wide,  but  in  the  photograph  the  width  is  represented 
as  greater  than  the  height.  These  two  examples  are  not 
at  all  such  extreme  cases  as  might  have  been  selected, 
but  they  serve  to  show  how  careful  a  photographer 
should  be  not  to  include  too  great  an  angle  of  view. 
This  kind  of  distortion,  as  in  the  previous  case,  disappears 
entirely  if  the  photograph  is  looked  at  with  one  eye 
from  the  correct  view  point,  and  indeed  is  then  neces¬ 
sary  in  order  to  properly  represent  the  objects.  But  as 
photographs  are  not  generally  examined  under  such 
restrictions,  it  is  an  error  to  produce  pictures  that  need 

them  in  order  to  appear  correct. 

The  above  errors  relate  to  ‘‘drawing”  or  shape.  In 
the  previous  chapter  we  have  referred  to  the  effects  of 
colour  and  its  reproduction,  and  the  only  remaining 
item  in  photography  is  the  gradation,  or  the  repioduction 
of  light  and  shade.  Here  also  error  is  possible,  and  the 
statement  that  was  made  some  time  ago  by  a  misled 
enthusiast,  that  the  character  of  a  photograph  is  settled 
as  soon  as  the  cap  is  put  on  the  lens  (that  is,  at  the 
conclusion  of  the  exposure),  is  not  coriect,  although  it 
has  often  been  quoted  as  authoritative.  The  exposure 
has  something  but  not  everything  to  do  with  this 
matter,  it  is  affected  by  other  circumstances  too.  The 
author  has  had  to  do  with  a  case  in  which  the 
vital  question  was  as  to  the  visibility  of  things  throu^,  i 

271 


Truth  and  Error  in  Photography 

a  certain  mist  or  smoke  and  whether  this  was  truly  or 
deceptively  represented  in  certain  photographs.  In  such 
a  case  the  photograph  may  be  true  or  false.  The  mist 
itself  is  represented  in  the  print  by  a  patch  of  blankness, 
or  very  nearly  that,  and  whatever  is  seen  through  it  is 
seen  feeble  and  white  in  proportion  to  the  opacity  of  the 
mist.  If  the  negative  is  under-exposed  or  made  with  too 
much  contrast,  or  if  the  print  is  under-exposed,  then  the 
objects  visible  through  the  mist  or  smoke  will  not  show 
as  clearly  as  they  should,  or  they  may  not  show  at  all, 
and  thus  the  opacity  or  bulk  of  the  mist  will  appear  in 
the  print  to  be  greater  than  it  really  is.  The  test  in  this 
case  also  is  to  compare  the  photograph  with  the  thing  | 
that  it  represents,  and  to  see  that  it  is  of  such  a  nature  j 
that  one  who  has  not  seen  the  object  itself  would  be 
able  from  the  photograph  to  imagine  it  correctly. 

It  should  be  noticed  that  what  we  have  called  errors 
are  nothing  more  than  the  natural  results  of  the  procedure  | 
that  leads  to  them,  that  they  are  generally  caused  by  a 
want  of  judgment  on  the  part  of  the  photographer,  and  j 
that  it  is  easy  to  know  in  what  direction  such  errors 
are  possible  or  likely.  There  are  other  errors  possible, 
due  to  imperfections  in  the  materials  used.  A  bubble 
in  the  emulsion  coating  on  the  plate  causes  a  deficiency  j 
of  emulsion  at  that  point,  and  therefore  a  thin  place  , 
and  a  dark  spot  on  the  print.  Opaque  specks  of  dust  on 
the  plate  will  give  white  spots  on  the  print.  These  may 
seem  trivial  faults  and  in  ordinary  cases  aie  readily 
removed  by  a  little  Indian  ink  or  water-colour,  spotting 
as  it  is  called  ;  but  if  the  subject  itself  consists  of  spots, 
such  as  a  photograph  of  the  stars,  a  fault  of  this  kind 
may  lead  to  uncertainty.  There  can  be  no  certainty  that 
such  faults  are  ever  eliminated,  for  the  air  is  full  of 
particles ;  if  therefore  the  presence  or  absence  of  a  small 
spot  is  a  vital  matter  in  a  photograph,  two  or  perhaps 
three  negatives  must  be  taken.  A  spot  that  is  due  tor 

272 


Truth  and  Error  in  Photography 

the  object  photographed  will  appear  on  all  the  plates, 
but  the  probability  that  an  accidental  spot  of  the  same 
character  should  happen  to  come  in  exactly  the  same 
place  on  two  or  three  different  plates  is  extremely  remote. 
Other  errors  are  possible  due  to  irregularities  in  the 
emulsion  or  the  coating,  such  as  a  difference  between 
the  edges  of  the  plate  and  the  middle,  and  while  these 
are  not  frequent  and  rarely  obtrusive  in  ordinary  work, 
for  more  exact  purposes  the  possibility  of  them  has  to 
be  borne  in  mind. 

We  have  already  remarked  that  no  lens  can  be  perfect. 
We  want  all  the  light  that  falls  upon  a  lens  to  pass  through 
it  and  so  to  form  as  bright  an  image  as  possible,  but 
when  light  falls  upon  a  glass  surface  some  of  it  is  always 
reflected,  and  if  the  reflected  light  simply  passed  away 
and  was  lost  there  would  be  but  little  trouble,  as  it  would 
mean  at  most  a  little  longer  exposure  to  make  up  for 
the  lost  light.  But  lens  surfaces  are  curved  and  the 
reflected  light  is  sent  from  one  to  another,  and  the  curved 
surfaces  act  as  mirrors  and  form  other  images  than  the 
main  one.  By  holding  a  lens  at  a  little  distance  from 
the  eye  and  looking  through  it  at  a  candle  or  gas  flame, 
a  number  of  these  images  produced  by  reflection  will 
be  seen,  six  if  the  lens  is  of  the  “rapid  rectilinear”  type, 
and  fifteen  in  the  common  type  of  portrait  lens  :  more 
in  the  latter  case,  because  there  are  six  lens  surfaces 
instead  of  four  as  in  the  rapid  rectilinear.  This  reflection 
cannot  be  got  rid  of,  so  the  optician  aims  at  getting  the 
images  formed  by  it  far  from  the  plate  that  they  may  be 
as  much  out  of  focus  as  possible,  and  the  light  spread 
as  far  as  possible  over  the  whole  plate.  Success  in  this 
attempt  varies,  and  in  the  use  of  lenses  for  critical  or 
scientific  work  this  interfering  circumstance  must  not  be 
|  lost  sight  of.  The  amount  of  light  reflected  is  only  a 
small  proportion  of  the  whole  ;  these  false  or  ghost  images 
are  therefore  comparatively  feeble,  but  if  any  part  of  the 

273  s 


Truth  and  Error  in  Photography 

chief  image  is  very  bright,  they  may  be  obtrusive  even 
to  the  casual  observer.  The  author  has  a  photograph 
of  a  gentleman  in  evening  dress  in  front  of  a  grey  back¬ 
ground,  taken  by  a  professional  photographer  in  the 
ordinary  way  of  business,  and  above  his  head  there  is 
a  triangular  patch  of  light  which  was  a  source  of  annoy¬ 
ance  and  mystification  to  several  photographers.  This 
patch  is  nothing  else  than  a  ghost  image,  produced  by 
reflection  from  the  lens  surfaces,  of  the  man’s  shirt  front. 
This  explanation  of  the  patch  of  light  was  not  readily 
accepted  at  the  time,  but  investigation  proved  that  it  was 
correct. 

This  reflection,  instead  of  giving  a  subsidiary  or  ghost 
image  of  a  bright  portion  of  the  object  (of  course  an 
image  of  the  whole  object  is  formed,  but  the  image  of 
the  less  bright  parts  is  generally  too  feeble  to  produce 
any  noticeable  effect),  may  give  an  image  of  the  diaphragm 
of  the  lens,  if  one  of  its  reflected  images  is  approximately 
in  focus  on  the  plate.  This  image  will  be  always  in  the 
middle  of  the  plate,  that  is  opposite  to  the  lens,  because 
the  diaphragm  is  central  to  the  lens.  It  will  probably  be 
more  obtrusive  when  the  diaphragm  has  a  small  aperture 
than  when  it  is  opened  out  to  a  larger,  indeed  in  the 
latter  case  it  may  be  so  diffused  as  to  disappear.  These 
central  spots  are  called  “flare  spots.” 

We  have  not  described  all  the  unwelcome  and  inter¬ 
fering  effects  that  photography  is  liable  to,  but  we  have 
said  enough  to  show  that  the  photography  of  even  common 
objects  is  not  the  simple  matter  that  so  many  imagine  it  to 
be.  And  when  we  come  to  critical  or  scientific  work,  it  is 
easy  to  see  that  the  most  careful  work  by  one  who  has  not 
seriously  studied  the  subject  may  be  not  only  insufficient 
but  actually  false  and  misleading.  This  fact  is  rarely 
appreciated  by  those  to  whom  the  public  looks  for  guidance 
in  educational  matters,  and  even  those  whose  work 
depends  upon  photography  generally  neglect  it  whether 

274 


Truth  and  Error  in  Photography 

they  are  cognisant  of  it  or  not.  Indeed  we  may  go  further 
and  say  that  those  whose  livelihood  depends  upon  photo¬ 
graphy  often  do  not  appreciate  it,  except  to  a  certain  extent 
now  and  then,  when  they  suffer  inconvenience  and  it  may 
be  much  pecuniary  loss  that  might  have  been  avoided  if 
they  had  taken  the  trouble  to  see  that  their  procedure  was 
founded  on  true  principles. 


275 


CHAPTER  XVII 


INSTANTANEOUS  PHOTOGRAPHY  AND  THE 
PHOTOGRAPHY  OF  MOTION 

THE  words  instant  and  moment  are  very  useful  though 
very  indefinite.  We  have  been  told  that  there  is  no  such 
thing  as  an  instantaneous  exposure  in  photography  be¬ 
cause  every  exposure  is  of  a  measurable  duration,  but  it  is 
not  usual  to  restrict  the  word  instantaneous  to  the  meaning 
of  a  geometric  point  of  time,  and  indeed  if  the  word  were 
so  restricted  we  should  have  no  practical  use  for  it.  We 
generally  regard  about  the  tenth  of  a  second  as  the  longest 
exposure  that  we  should  call  instantaneous,  and  there  is 
a  good  reason  for  this,  because  it  is  about  the  shortest 
period  for  which  it  is  possible  for  us  to  see  anything. 
When  we  shut  our  eyes  we  do  not  immediately  cease 
to  see  what  we  were  looking  at,  and  if  we  shut  and  open 
our  eyes  as  quickly  as  we  can,  vision  remains  continuous 
although  the  light  was  stopped  for  a  moment  from  enter¬ 
ing  our  eyes.  The  effect  on  the  eye,  or  we  may  say  the 
picture  produced  on  the  retina,  lasts  for  about  the  tenth 
of  a  second,  so  that  if  we  look  at  an  object  that  is  illumi¬ 
nated  by  an  electric  spark  that  lasts  for  only  the  millionth 
part  of  a  second,  although  the  actual  image  of  the  object 
was  on  the  retina  for  only  this  minute  period,  we  see  the 
object  for  about  the  tenth  of  a  second  because  the  effect 
on  the  retina  persists  for  so  long.  And  if  the  illumination 
lasted  for  a  thousand  millionths  of  a  second,  we  should 
still  see  it  for  about  the  same  time— we  continue  to  see  it 
after  it  has  ceased  to  be  visible  until  the  effect  it  has  pro- 

276 


Instantaneous  Photography 

duced  has  died  away.  It  may  be  thought  impossible  to 
see  a  thing  after  it  has  ceased  to  be  visible,  or  ceased  to 
exist  (as  in  the  case  of  the  spark  itself),  but  it  was  pointed 
out  in  an  early  chapter  that  some  of  the  stars  that  we  see 
may,  so  far  as  we  can  tell,  have  ceased  to  shine  thousands 
of  years  ago,  for  we  are  so  much  behindhand  in  our  observa¬ 
tions  because  of  the  time  that  it  takes  light  to  travel  over 
so  great  a  distance  as  that  which  separates  us  from  them. 
That  we  really  see  things  after  they  have  passed  away  is 
commonly  illustrated  by  swinging  a  lighted  taper  or  any¬ 
thing  of  the  sort,  when  the  light  becomes  a  streak  that  is 
longer  the  more  quickly  the  taper  is  moved,  because  we  not 
only  see  the  flame  where  it  is,  but  also  where  it  was  about 
the  tenth  of  a  second  before,  and  in  all  the  intermediate 
positions.  We  shall  see  the  importance  of  this  “  persist¬ 
ence  of  vision "  in  the  later  part  of  this  chapter,  mean¬ 
while  it  gives  us  a  key  to  the  meaning  of  the  word 
instantaneous.  There  is  another  reason  why  photo¬ 
graphers  generally  adopt  such  a  meaning,  and  that  is  that 
the  tenth  of  a  second  is,  in  shortening  the  exposure,  the 
first  exposure  that  requires  a  contrivance  to  give  it.  By 
removing  and  replacing  the  lens  cap  as  quickly  as  possible 
by  hand,  the  exposure  given  is  equal  to  about  the  quarter 
or  fifth  of  a  second,  and  for  anything  shorter  than  this  we 
must  have  a  “  shutter,"  or  some  equivalent  device. 

From  the  earliest  times  of  the  Daguerreotype,  when 
persons  would  sometimes  have  their  faces  whitewashed 
in  order  to  shorten  the  tedious  exposure  by  a  few  minutes, 
the  aim  has  been  to  reduce  the  period  of  the  exposure 
necessary.  With  ordinary  lenses  portraits  now  need  ex¬ 
posures  of  only  a  second  or  two  indoors  and  a  mere  fraction 
of  a  second  out-of-doors.  We  may  roughly  say  that  the 
most  rapid  exposure  that  will  serve,  as  of  performing 
athletes,  out-of-doors  with  the  best  summer  light  and 
other  advantageous  circumstances,  is  about  the  one  five- 
hundredth  part  of  a  second.  Such  an  exposure  is  rarely 

277 


Instantaneous  Photography 

given,  even  by  those  who  have  apparatus  marked  to  give 
it,  but  we  may  consider  it  as  the  practical  limit..  If  we 
dispense  with  the  camera  and  get  a  powerful  electric  spark 
to  shine  directly  upon  the  plate,  the  object  to  be  “photo¬ 
graphed”  coming  between  the  plate  and  the  light  so  that 
it  casts  a  shadow  on  the  plate,  the  limit  at  present  is 
approximately  the  one-millionth  part  of  a  second. 

The  exposures  just  mentioned  may  be  regarded  as  the 
very  shortest ;  the  great  majority  of  instantaneous  exposures 
come  between  the  tenth  or  fifteenth  of  a  second  and  the 
fiftieth  of  a  second,  for  although  many  shutters  are  marked 
to  give  the  one-hundredth  part  of  a  second,  a  very  great 
number  of  them  when  set  to  this  figure  do  not  give  a 
shorter  exposure  than  the  thirtieth  or  fortieth  of  a  second. 
The  exposure  necessary  to  produce  a  proper  effect  upon 
the  plate  is  calculated  in  exactly  the  same  way  as  when 
longer  exposures  are  given,  and  it  can  be  shortened,  while 
remaining  effective,  only  by  increasing  the  aperture  of 
the  lens  or  getting  a  more  sensitive  plate.  By  choosing 
a  brighter  day,  and  if  one  is  photographing  an  athlete, 
having  him  dressed  in  white  rather  than  black  or  red,  an 
getting  on  the  brighter  side  of  him  if  the  sun  shines,  will 
help  to  get  a  sufficient  exposure  effect  in  a  shorter  time. 
If  because  of  the  rapid  movement  of  the  object  the  exposure 
has  to  be  shorter  than  will  give  a  “properly  exposed 
plate,  then  the  under-exposure  must  be  accepted  and 
made  the  best  of.  Here  there  is  room  for  special  skill 
and  experience. 

Photographers  talk  glibly  about  short  periods  of  time, 
such  as  the  one-liundredth  of  a  second,  and  while  this  is 
a  perfectly  definite  period,  when  applied  to  a  photographic 
exposure  it  may  mean  three  different  things,  because  there 
is  no  shutter  or  other  mechanism  that  will  open  in  no 
time,  remain  open  for  the  period  required  and  close  in 
no  time.  Some  of  the  total  time  is  taken  up  in  the  opening 
and  closing,  and  at  these  times  the  lens  is  not  fully  open. 

278 


Instantaneous  Photography 

If  the  shutter  is  arranged  as  a  blind  with  an  opening  that 
is  drawn  across  the  front  of  the  plate,  “  focal  plane  shutters  ” 
they  are  called,  the  same  considerations  apply.  The  open¬ 
ing  begins  at  zero  and  gradually  increases  to  the  full,  then 
gradually  decreases  until  it  is  closed.  The  proportion 
between  the  total  time  occupied  and  the  time  that  would 
be  necessary  to  produce  the  same  effect  on  the  plate 
if  no  time  were  occupied  in  the  opening  and  closing,  is 
called  the  “  efficiency ”  of  the  shutter.  When  a  short 
time,  such  as  the  hundredth  of  a  second,  is  stated  in  this 
connection,  it  may  refer  to  any  period  between  the  total 
duration  of  the  exposure,  and  the  shorter  time  that  would 
produce  the  same  effect  if  the  lens  were  fully  open  all 
the  while.  The  proportion  between  these  two  varies 
very  much  according  to  the  instruments  themselves  and 
also  according  to  the  conditions  under  which  they  are 
used. 

The  advantage  of  being  able  to  secure  a  photograph 
in  such  short  periods  as  those  we  have  been  discussing, 
is  that  moving  objects  may  be  photographed  while  they 
have  time  to  move  to  so  small  an  extent  that  the  photograph 
is  not  blurred  beyond  a  negligible  amount.  The  faster  an 
object  is  moving  the  shorter  must  be  the  exposure,  but 
the  mistake  is  often  made  of  considering  only  the  rate  of 
progression  of  the  object.  A  man  may  be  walking  at 
four  miles  an  hour,  but  each  foot  will  move  at  consider¬ 
ably  more  than  eight  miles  an  hour  when  it  is  at  the 
middle  of  its  swing.  If  the  photographer  is  skilful  enough 
to  expose  at  the  moment  when  both  the  man’s  feet  are  on 
the  ground,  then  the  more  rapid  movement  may  be  ignored. 
In  many  other  cases  there  are  advantageous  moments  of 
this  kind  that  will  permit  of  a  longer  exposure  being  given 
than  if  the  most  rapid  movement  of  the  subject  were 
allowed  for. 

Instantaneous  exposures  may  be  given  with  any  kind  of 
camera,  and  it  is  often  of  great  advantage  to  have  the 

279 


Instantaneous  Photography 

camera  firmly  supported  on  a  stand.  But  one  reason  for 
desiring  short  exposures  is  that  the  stand  may  be  dispensed 
with  and  the  camera  merely  held  in  the  hand  while  the 
exposure  is  given,  the  movement  of  the  person  holding  it 
becoming  negligible  by  reason  of  the  shortness  of  the  ex¬ 
posure.  Most  persons  will  not  move  a  camera  far  enough 
to  affect  the  result  in  the  twentieth  of  a  second  if  they  try 
to  hold  it  still  ;  with  practice  this  may  be  extended  to  the 
tenth  of  a  second,  and  with  a  few  persons  to  the  fifth. 
Cameras  made  for  use  in  this  way  are  generally  called 
u  hand  cameras,”  but  of  course  if  there  is  a  convenient 
post,  tree,  window  ledge,  wall  or  other  suitable  object 
upon  which  the  camera  may  be  rested,  the  wise  photo¬ 
grapher  will  avail  himself  of  such  support. 

The  great  convenience  of  having  a  camera  in  the  hand, 
ready  at  a  moment’s  notice,  as  compared  with  a  camera 
that  has  to  be  got  ready,  set  up  and  mounted  on  a  stand, 
must  have  been  obvious  from  the  first,  but  until  gelatine 
dry  plates  were  commercially  produced  such  an  apparatus 
was  not  worth  troubling  about,  because  the  plates  avail¬ 
able  were  not  sufficiently  sensitive  to  make  them  effective. 
In  1881,  Mr.  Thomas  Bolas  devised  several  “detective” 
cameras,  as  they  were  called,  presumably  because  of 
their  possible  use  to  detectives  in  getting  photographic 
evidence  that  might  be  useful  to  them  when  the  cases  they 
were  investigating  came  into  court.  For  some  years  this 
name  remained  in  use,  and  the  idea  was  to  have  a  camera 
that  could  not  be  recognised  as  such,  so  that  photographs 
might  be  taken  in  the  street  or  in  other  public  places 
without  the  knowledge  of  the  passers-by  or  the  persons 
being  photographed.  A  common  arrangement  was  to 
have  the  camera  outwardly  resembling  a  brown  paper 
parcel,  and  later  on  it  was  concealed  under  the  waistcoat, 
the  lens  taking  the  place  of  one  of  the  buttons,  or  under 
the  tie  with  the  hope  that  the  lens  might  be  mistaken  for 
the  head  of  the  tie-pin.  But  as  the  number  of  such  secret 


Instantaneous  Photography 

cameras  increased  the  more  rapidly  were  they  recognised, 
and  adding  to  this  fact  the  necessary  attention  of  the 
photographer  to  his  apparatus  and  the  special  movements 
necessary  to  make  an  exposure,  there  is  good  reason  to 
doubt  whether  “  detective”  cameras  ever  really  served  their 
purpose  in  secret. 

In  giving  up  the  notion  of  secrecy,  the  way  was 
opened  to  consider  convenience  only,  and  this  has  led  to 
the  introduction  of  innumerable  patterns  of  hand  cameras. 
Some  of  them  fold  up  and  are  therefore  more  convenient 
for  carrying,  but  need  more  preparation  before  they  are 
ready  for  an  exposure  than  the  box  form  in  which  the 
lens  and  plate  are  always  in  position  ready  for  use.  Of 
course  a  camera  intended  for  use  on  a  stand,  an  ordinary 
“field”  camera,  if  it  has  a  shutter  on  the  lens,  can  be 
used  as  a  hand  camera,  and  some  excellent  large  photo¬ 
graphs  of  yachts  have  been  taken  in  this  way,  but  such 
cameras  are  often  inconvenient  to  hold,  and  if  pressed 
against  the  body  to  keep  them  steady,  the  focussing  ad¬ 
justment  is  liable  to  be  interfered  with.  And  almost  all 
hand  cameras  may  be  fixed  upon  stands,  so  that  there  is 
no  radical  difference  between  the  two  types. 

In  the  use  of  a  hand  camera,  it  is  desirable  if  not 
always  necessary  to  have  a  special  means  of  knowing 
when  the  image  of  the  object  to  be  photographed  is 
properly  in  position,  so  that  it  will  fall  upon  the  plate  as 
desired  when  the  shutter  is  opened.  If  the  object  is 
moving  it  may  be  necessary  to  follow  it  so  that  the  shutter 
may  be  released  at  exactly  the  time  when  it  has  arrived 
at  the  position  desired.  A  great  deal  of  ingenuity  has 
been  expended  on  the  construction  of  “finders.”  One  of 
the  commonest  is  merely  a  small  camera  which  is  fixed  to 
the  main  camera  and  gives  a  picture  about  the  size  of  a 
postage  stamp,  the  same  that  will,  on  a  larger  scale,  fall 
upon  the  plate.  The  small  picture  can  be  watched  all  the 
time.  The  “  reflex  ”  form  of  camera  is  a  favourite  with 

281 


Instantaneous  Photography 

many  in  spite  of  its  bulkiness,  because  this  gives  a  full 
size  picture  of  the  object  that  can  be  watched  up  to  the 
moment  of  exposure.  Only  one  lens  is  used,  and  a  mirror 
behind  it  reflects  the  image  up  to  the  ground  glass  at  the 
top  of  the  camera  until  the  shutter  release  is  touched, 
then  the  mirror  moves  out  of  the  way  immediately  before 
the  shutter  gives  the  exposure. 

What  is  the  particular  use  of  a  hand  camera  ?  Pro¬ 
bably  the  greater  number  are  only  toys  in  the  hands  of 
their  owners,  and  used  for  making  snapshots  of  anything 
that  happens  to  strike  their  fancy.  It  may  seem  surprising 
at  first  that  so  great  a  number  of  the  photographs  taken 
in  such  a  haphazard  way  are  successful,  and  it  is  some¬ 
times  argued  from  this,  by  those  who  know  nothing  or 
little  about  the  matter,  that  photography  as  an  art  does 
not  need  serious  study.  The  measure  of  success  in  this 
direction,  whatever  it  may  be,  is  due  largely  to  the  fact 
that  those  who  merely  “press  the  button"  generally  use 
their  cameras  in  fine  weather  and  on  subjects  that  require 
very  similar  exposures.  It  is  also  due  largely  to  the  fact 
that  those  who  supply  them  with  cameras,  plates,  films,  and 
printing  papers,  know  their  habits  and  provide  them  with 
materials  that  will  best  suit  the  treatment  that  they  know 
they  are  likely  to  receive.  In  short,  we  may  say  that  all 
the  photography  is  done  for  them,  and  they  only  follow 
a  few  simple  instructions.  This  snap-shotting  is  a  form  | 
of  amusement  that  deserves  commendation,  because  it  j 
is  generally  interesting  and  harmless,  it  does  a  little  to-  j 
wards  training  the  powers  of  observation  of  those  who 
practise  it,  and  it  may  lead  them  to  engage  in  more  , 
useful  work. 

But  hand  cameras  are  also  largely  used  for  the  getting  > 
of  photographic  records  under  conditions  that  would 
render  it  very  inconvenient  if  not  impossible  to  fix  up  a 
stand.  The  photographs  of  current  events  that  we  have 
become  accustomed  to  expect  in  daily  papers,  the  photo- J| 

282 


Instantaneous  Photography 

graphs  of  living  things  of  all  kinds,  animals,  birds,  insects, 
reptiles,  and  fishes,  that  are  rapidly  rendering  obsolete  the 
pictorial  representations  of  them  that  we  were  accustomed 
to  a  generation  ago,  and  innumerable  other  subjects  both 
indoors  and  out-of-doors,  are  often  more  conveniently 
photographed  with  a  hand  camera,  either  because  of  its 
portability,  the  rapidity  with  which  it  can  be  got  ready 
for  use,  or  its  special  adaptations,  for  their  use  is  not 
confined  to  subjects  that  need  very  short  exposures. 

The  photographic  shutter  acts  by  admitting  the  light 
to  the  lens  and  so  to  the  plate  for  the  necessary  period,  but 
if  the  light  itself  exists  for  only  the  desired  period  there 
is  no  need  for  the  shutter.  The  lens  may  be  uncovered 
shortly  before  the  light  comes  and  closed  after  the  ex¬ 
posure.  This  is  the  method  of  photographing  lightning, 
but  as  one  cannot  tell  when  the  flash  will  come,  the  lens 
must  be  opened  and  pointed  towards  that  part  of  the  sky 
where  it  is  most  likely  to  come.  The  duration  of  the  light 
determines  the  duration  of  the  exposure.  The  same  con¬ 
dition  may  hold  with  artificial  lights,  in  the  use  of  which 
it  is  often  more  convenient  to  regulate  the  exposure  by 
the  light  rather  than  by  the  shutter.  By  blowing  finely 
powdered  magnesium  (or  aluminium)  through  the  flame  of 
a  spirit-lamp  a  bright  flash  is  obtained,  and  the  brilliancy 
of  the  light  may  be  roughly  regulated  by  the  quantity  of 
powdered  metal  used.  This  is  a  common  method  of 
illumination  when  photographing  guests  at  dinners  or 
other  functions,  and  by  india-rubber  tubing  several  flash 
lamps,  each  containing  its  dose  of  magnesium,  may  be 
actuated  simultaneously  and  a  large  hall  may  be  suf¬ 
ficiently  illuminated.  A  flash  of  this  kind  will  probably 
range  from  about  a  quarter  of  a  second  up  to  a  second 
or  so  according  to  the  quantity  of  the  metal  and  the 
suddenness  of  the  puff  that  blows  the  metal  into  the 
flame. 

A  much  more  rapid  and  also  brighter  flash  may  be 

283 


Instantaneous  Photography 

obtained  by  mixing  the  powdered  magnesium  with  some 
substance  that  contains  the  oxygen  necessary  for  its  com¬ 
bustion  instead  of  simply  allowing  it  to  burn  in  the  air. 
Such  a  mixture  is  analogous  to  gunpowder  and  should 
never  be  prepared  or  used  by  anyone  who  does  not  know 
how  to  handle  explosives.  Of  these  mixtures  some  are 
more  dangerous  than  gunpowder,  and  others  that  are  less 
risky  have  sometimes  been  the  cause  of  deplorable  acci¬ 
dents.  If  carefully  prepared,  a  few  grains  of  one  of  these 
mixtures  will  give  a  flash  that  lasts  only  about  the  twenty- 
fifth  of  a  second.  But  by  a  suitable  arrangement  a  bright 
electric  spark  may  be  obtained  that  lasts  for  only  about 
the  millionth  part  of  a  second,  a  duration  that  it  is  im¬ 
possible  to  conceive.  In  the  millionth  of  a  second,  a 
bullet  from  a  rifle  has  scarcely  time  to  move  a  perceptible 
amount,  and  by  arranging  for  a  bright  spark  that  the 
bullet  itself  will  cause  to  pass  by  completing  the  contact 
of  the  electric  wires,  and  fixing  the  apparatus  so  that  the 
bullet  as  it  flies  passes  between  the  spark  and  the  plate, 
the  plate  will  be  made  developable  except  where  the 
shadow  of  the  bullet  falls  upon  it.  This  is  the  way  that 
allows  one  to  photograph  a  bullet  travelling  at  full  speed. 
Drops  of  water  as  they  fall,  soap  bubbles  as  they  burst,  and 
other  rapidly  moving  objects,  have  had  the  nature  of  their 
movements  ascertained  by  similar  means. 

Closely  connected  with  the  photography  of  moving 
bodies  is  the  photography  of  motion,  by  which  we  mean 
the  photography  of  a  moving  body  at  various  regular 
periods  of  its  movement.  Having  such  a  series  of  photo¬ 
graphs  arranged  in  order,  by  running  the  eye  along  them 
we  see  the  various  stages  of  the  movement,  and  by  letting 
our  imagination  fill  in  the  gaps,  we  get  a  very  good  idea 
of  its  character.  It  is  better  still  if  such  a  series  can  be 
put  in  an  apparatus,  which  as  a  toy  used  to  be  called  a 
“  zoetrope  "  or  “wheel  of  life,"  because  when  it  is  rotated 
the  succeeding  pictures  rapidly  take  the  place  of  each 

284 


Edgar  Scamell 

The  Like  History  of  a  Nasturtium 

30th  May  6th  June 

1 6th  J  une  6lh  J  uly 

These  and  those  on  the  succeeding  plate  are  selected  from  a  series  of  nearly  a  hundred 
photographs,  taken  one  every  day  during  the  life  of  the  plant,  --Uch  a  series  arranged  in  a 

(See page  opposite.) 


Edgar  Scamell 

The  Life  History  of  a  Nasturtium 

16th  July  23rd  July 

2nd  August  22nd  August 

suitable  cinematograph  apparatus  will  show  in  a  few  seconds  the  movements  of  the  plant 
during  its  development  that  in  fact  were  spread  over  a  period  of  about  three  months. 

( See  page  opposite .) 


Instantaneous  Photography 

other,  with  a  little  interval  between  which  is  too  short  for 
the  eye  to  recognise.  The  illusion  of  movement  is  then 
complete.  The  subjects  shown  in  such  apparatus  are 
generally  in  rapid  motion,  such  as  running  horses  and 
jumping  men,  but  the  instrument  is  available  for  the  in¬ 
vestigation  of  movement  of  any  sort,  and  if  the  photo¬ 
graphs  can  be  secured,  a  very  rapid  movement  may  be 
slowly  reproduced  and  its  details  clearly  seen,  or  a  very 
slow  movement  may  be  shown  more  quickly  that  its 
character  may  be  more  easily  appreciated.  As  an  illustra¬ 
tion  of  the  latter,  Mr.  Edgar  Scamell  a  few  years  ago 
photographed  a  nasturtium  plant  every  day  for  eighty  days, 
beginning  when  the  plant  first  appeared  above  the  ground 
and  continuing  until  it  was  fully  grown,  and  the  seeds 
ripe  and  falling  from  their  vessels.  The  growth  of  a  plant 
is  so  slow  that  it  is  impossible  by  the  unaided  eye  to  trace 
it,  but  by  showing  such  a  series  of  photographs  in  the  way 
described,  the  whole  process  may  be  seen  as  if  it  occupied 
only  a  few  seconds  instead  of  eighty  days.  The  following 
are  the  chief  dates  of  the  progress  :  2nd  May,  seed  planted; 
2 ist  May,  first  appearance  above  the  soil ;  16th  July,  buds 
formed;  23rd  July,  flowers  opening;  2nd  August,  seed 
formed ;  22nd  August,  seed  fully  grown  and  fell.  By  the 
kindness  of  Mr.  Scamell  eight  of  this  series  are  reproduced, 
and  they  will  serve  to  show  the  general  nature  of  the 
whole. 

But  it  is  chiefly  with  regard  to  rapidly  moving  objects 
that  the  photography  of  motion  is  associated,  for  here  the 
eye  cannot  see  the  various  positions  of  the  moving  object, 
and,  judging  only  from  the  general  effect  of  the  movement, 
the  spectator  is  often  very  much  deceived  as  to  its  details. 
If  not  the  very  first  work,  the  first  successful  work  in  this 
direction  appears  to  have  been  due  to  Mr.  Muybridge  of 
California,  and  was  undertaken  by  him  in  1878  or  1879 
in  order  to  decide  a  wager.  The  point  in  dispute  was  as 
to  whether  a  trotting  horse  at  any  stage  of  his  movement 

285 


Instantaneous  Photography 

had  all  four  feet  off  the  ground  at  the  same  time.  Mr. 
Muybridge  arranged  a  platform  along  which  the  horses 
were  to  trot,  with  a  series  of  cameras  fitted  with  rapid 
lenses  and  quick  shutters,  and  arrangements  by  which  the 
horse  automatically  released  the  shutter  as  he  passed  in 
front  of  each  camera.  This  apparatus  was  subsequently 
improved,  and  Mr.  Muybridge  devoted  himself  for  many 
years  to  an  analysis  of  the  method  of  progression  of 
various  animals,  including  human  beings.  Similar  investi¬ 
gations  were  carried  out  by  M.  Marey  of  Paris,  but  with 
a  simpler  apparatus  and  one  that  would  permit  of  dealing 
with  movements,  such  as  jumping,  that  do  not  include 
a  sufficient  change  of  position  to  conveniently  bring  the 
subject  in  front  of  another  camera.  He  started  work  in 
1882  and  used  a  single  camera,  and  by  a  rotating  shutter 
that  rapidly  opened  and  closed  the  lens  alternately,  he 
obtained  a  large  number  of  photographs  of  the  subject  on 
the  same  plate.  These  photographs  would  overlap,  but 
that  was  no  drawback,  indeed  sometimes  it  was  an  ad¬ 
vantage  in  tracing  the  rate  and  character  of  the  movement 
so  long  as  the  outline  or  any  conspicuous  part  was  visible 
to  represent  the  desired  part.  The  movement  of  the  foot 
of  a  jumping  man,  for  example,  could  easily  be  traced 
if  only  the  outline  of  the  heel  was  shown  in  the  photo¬ 
graph.  In  the  following  year  M.  Marey  constructed  a 
photo-physiological  studio,  with  chronographs,  scales,  and 
other  conveniences  for  exactly  registering  the  times  and 
distances  concerned.  In  order  to  get  more  precise  details, 
he  sometimes  dressed  his  subject  in  black  so  that  he  did 
not  show  against  the  black  background,  and  put  a  bright 
narrow  stripe  to  indicate,  for  example,  the  leg  and  foot. 
Thus  the  part  desired  was  represented  in  the  photograph 
by  a  mere  line,  and  this  enabled  one  hundred  instead  of 
only  ten  exposures  per  second  to  be  made  without  confusion. 


Instantaneous  Photography 

Later,  on  M.  Marey  photographed  the  movements  of  fishes 
swimming  in  a  tank  and  birds  flying  in  the  air,  as  well  as 
of  men  and  animals,  with  remarkable  success. 

A  simple  method  like  that  just  described  is  still  pro¬ 
bably  the  best  for  the  analysis  of  the  movements  of  living 
creatures,  but  obviously  its  applications  are  purely  scientific, 
for  the  movement  cannot  be  reproduced  upon  the  screen 
or  in  a  “  wheel  of  life,”  because  the  separate  photographs 
overlap.  And  there  was  always  a  popular  side  to  this  kind 
of  work,  for  it  was  realised  that  if  a  series  of  photographs 
could  be  taken  with  sufficient  rapidity  and  shown  upon  the 
screen  like  lantern  slides,  also  with  sufficient  rapidity,  that 
instead  of  the  picture-like  stillness  of  the  figures  on  the 
screen  as  shown  by  an  ordinary  slide,  the  figures  would 
appear  to  move  as  in  reality,  and  we  should  have 
11  animated  photographs.”  It  was  possible  to  make  films 
or  plates  sensitive  enough  for  the  purpose  when  used  in 
conjunction  with  specially  rapid  lenses,  so  that  the  actual 
difficulty  was  a  mechanical  one.  It  is  necessary  for  this 
purpose  to  make  a  continuous  series  of  photographs,  about 
sixteen  every  second,  and  to  go  on  at  this  uniform  rate 
until  the  movement  is  completed  or  a  sufficient  record  is 
obtained.  It  is  easy  to  open  and  close  the  lens  at  this  rate 
by  the  use  of  a  disc  that  rotates  in  front  of  it  and  has 
openings  or  gaps  in  it.  But  the  movement  of  the  strip 
of  film  is  not  so  simple.  It  is  not  enough  to  simply  draw 
this  along  continuously,  for  in  that  case  every  picture 
would  be  taken  on  a  moving  film  and  be  blurred  in 
consequence.  The  film  must  move  while  the  lens  is 
covered,  remain  stationary  for  the  fraction  of  a  second 
that  it  is  uncovered,  then  move  on  again,  so  that  it  must 
be  as  it  were  jerked  on  exactly  the  correct  distance  about 
sixteen  times  a  second,  and  always  remain  perfectly  still 
while  the  lens  is  open.  And  these  movements  must  be 

287 


Instantaneous  Photography 

so  smooth  that  the  apparatus  itself  remains  perfectly 
steady.  If  it  were  to  vibrate,  as  many  motor  vehicles  do 
when  standing  with  the  engine  running,  the  pictures  would 
be  blurred  and  useless.  Every  one  knows  that  these 
difficulties  have  been  overcome,  for  “  living  pictures  ”  can 
be  seen  in  every  large  town. 

This  strip  of  photographs,  which  may  measure  any¬ 
thing  up  to  about  five  hundred  feet,  is  a  negative.  A 
print,  that  is  another  strip  of  equal  size,  is  made  from  it 
by  exposure  and  development,  and  this  strip  is  passed 
through  the  projecting  lantern  under  the  same  conditions 
as  the  negative  was  taken  in  the  camera.  Those  who  have 
seen  cinematograph  displays  know  to  what  degree  of 
perfection  the  apparatus  has  been  brought. 

We  have  described  how  transparencies  for  use  as 
lantern  slides  can  be  made  “  in  natural  colours,"  but  the 
getting  of  cinematograph  views  in  natural  colours  is  a 
very  different  matter.  We  might  make  three  films,  one 
each  for  the  redness,  greenness,  and  blueness  of  the 
moving  subject,  and  we  might  perhaps  be  able  to  get 
these  three  films  to  run  so  exactly  together  as  they  were 
being  exposed,  that  any  discrepancy  would  not  be  notice¬ 
able.  We  could  make  three  prints,  and  by  the  use  of  three 
lanterns  or  a  triple  lantern  project  the  images  on  the 
screen  as  we  have  already  described  in  connection  with 
the  photography  of  colour.  But  for  this  triple  projection 
method  it  is  necessary  that  the  three  coloured  pictures 
shall  exactly  coincide  on  the  screen,  and  the  least  discern¬ 
ible  fault  in  this  matter  is  fatal.  It  is  possible  to  admirably 
succeed  in  this  superposition  when  the  transparencies 
and  the  whole  apparatus  is  quite  still,  but  to  have  three 
films  jerking  forward  sixteen  times  a  second  so  exactly 
together,  and  to  have  the  whole  apparatus  so  perfectly 
steady,  that  the  three  images  shall  always  be  exactly 


Instantaneous  Photography 

superposed  on  the  screen,  needs  a  mechanical  perfection 
in  the  apparatus  that  can  scarcely  be  hoped  for.  The 
most  persevering  attempts  made  so  far  have  signally 
failed.  It  must  be  borne  in  mind  that  the  single  cine¬ 
matograph  picture  is  only  about  the  size  of  a  postage 
stamp,  and  that  when  this  is  projected  on  the  screen 
the  amount  of  enlargement  is  very  much  greater  than 
is  usual  with  an  ordinary  lantern  slide,  and  that  all  errors 
and  accidental  movements  are  proportionately  magnified. 

In  1902  Mr.  G.  Albert  Smith,  in  conjunction  with  Mr. 
Charles  Urban,  set  himself  the  task  of  investigating  the 
proposed  methods  of  cinematography  in  natural  colours, 
and  of  further  working  along  the  most  promising  lines. 
It  was  about  four  years  ago  that  they  began  their  exhi¬ 
bitions  by  a  method  that,  if  it  is  not  scientifically  perfect, 
is  satisfying  to  the  eye,  and  this  is  really  all  that  is  sought 
for  and  all  that  is  necessary  for  popular  displays.  A 
single  film  is  used,  so  that  the  difficulty  of  getting  two 
or  more  pictures  to  simultaneously  coincide  upon  the 
screen  is  done  away  with  altogether.  The  colour  photo¬ 
graphs  alternate  on  the  same  strip  of  film.  It  is  easy 
to  see  that  if  the  red,  green,  and  blue  photographs  follow 
one  another  instead  of  being  projected  on  the  screen 
at  the  same  time,  that  they  must  follow  each  other  so 
quickly  that  the  three  pictures  are  on  the  retina  at  the 
same  time,  for  it  is  only  by  combining  them  that  the 
desired  colours  are  produced.  To  ensure  this,  the  photo¬ 
graphs  must  be  taken  and  shown  at  three  times  the  rate 
necessary  for  ordinary  cinematograph  pictures,  and  to 
represent  the  subject  for  the  same  time  the  film  must 
be  three  times  as  long.  To  modify  these  exacting  and 
costly  conditions  Mr.  G.  A.  Smith  endeavoured  to  reduce 
the  necessary  colours  to  two,  and  in  this  he  has  been 
practically  successful.  He  uses  an  orange  red  and  a 

289  T 


Instantaneous  Photography 


bluish  green  filter,  and  the  limitations  which  this  restriction 
imposes  are  found  to  be  negligible  for  this  purpose.  The 
pictures  are  therefore  taken  at  a  double  rate,  about  thirty- 
two  per  second,  and  the  two  colour  filters  arranged  on 
a  rotating  disc  come  alternately  in  front  of  the  lens.  A 
print  is  made  from  the  strip  in  the  usual  way,  and  this 
when  passed  through  the  bioscope  lantern  at  the  double 
rate  only  needs  the  two  alternating  colour  filters  in  front 
of  the  lens  in  addition  to  the  usual  apparatus. 

The  methods  of  using  the  bioscope  or  cinematograph 
refer  to  standard  instruments  which  are  constructed  so 


that  they  may  carry  any  standard  films,  and  the  method 
of  seeing  the  pictures  is  that  of  the  ordinary  optical  or 
projection  lantern.  Apparatus  has  been  devised  in  which 
the  series  of  photographs  is  taken  in  a  spiral  on  a  glass 
plate.  The  series  of  prints  has  been  arranged  as  leaves' 
in  a  little  book  which,  by  bending  it  with  the  thumb  at 


the  edges  so  that  the  leaves  are  released  one  at  a  time, 
brings  the  pictures  into  view  in  due  order.  And  there 
have  been  other  modifications  of  apparatus  and  methods 


that  need  not  be  referred  to. 

Cinematography  is  at  present  in  its  infancy,  so  that 
there  is  almost  as  much  interest  in  the  fact  that  it  is 
possible  to  represent  motion  and  in  the  methods  of  doing 
it  as  in  the  subjects  represented.  As  this  interest  appeals 
to  the  general  public,  many  of  the  subjects  are  trivial 
and  only  aim  at  being  amusing;  many  of  the  films  are 
photographs  of  made-up  scenes  arranged  to  imitate  railway 
accidents,  military  assaults,  and  other  exciting  and  tragic 
incidents ;  but  there  is  evidence  that  this  sort  of  thing 
is  having  to  make  way  for  the  representations  of  real 
events.  The  instructional  value  of  “living  pictures”  far 


exceeds  that  of  the  ordinary  lantern  slide  in  many  cases, 
and  this  fact  is  being  growingly  appreciated.  The  ad- 


290 


Instantaneous  Photography 

vantages  of  the  cinematograph  are  being  recognised  also 
for  the  purposes  of  investigation,  and  there  is  no  room 
for  doubt  that  in  due  time  the  apparatus  necessary  for 
its  practice  will  take  their  proper  places  as  scientific 
instruments,  as  the  “  magic”  lantern  did  about  a  genera¬ 
tion  ago,  and  the  microscope  rather  earlier  than  that. 


CHAPTER  XVIII 

ON  SIZE  AND  SCALE 

It  often  happens  that  the  photographer  working  under 
ordinary  conditions  finds  that  the  picture  of  the  object 
that  he  photographs  is  not  of  the  size  that  he  wishes  it 
to  be.  It  may  be  so  large  that  only  a  part  of  it  will  come 
upon  the  plate  he  is  using,  or  it  may  be  so  small  that  the 
greater  part  of  the  plate  is  wasted.  We  want  to  be  able 
to  photograph  things  of  all  sizes,  even  down  to  the  most 
minute,  but  a  picture  of  a  fly  produced  on  the  same  scale 
as  the  portrait  of  a  man  would  be  of  little  service.  We 
propose  to  devote  this  chapter  to  the  consideration  of 
the  control  of  the  size  of  the  image,  and  we  will  begin 
with  large  objects,  such  as  landscapes  and  buildings,  and 
pass  gradually  to  the  most  minute  which  are  far  too  small 
for  the  unaided  eye  ever  to  see. 

The  amateur  with  his  one  lens  does  not  set  up  his 
camera  a  hundred  yards  away  from  a  person  he  wishes 
to  photograph,  because  he  knows  that  at  that  distance  the 
picture  of  the  person  would  be  very  small.  He  comes 
nearer  and  nearer,  and  if  he  could  watch  the  image  on 
the  ground  glass  he  would  see  the  portrait  growing 
larger  at  every  step,  very  slowly  at  first,  but  more  rapidly 
as  he  approached  the  subject.  He  regulates  the  size  of 
the  image  by  altering  his  distance  from  the  object.  But 
every  one  who  knows  a  little  about  the  rules  of  per¬ 
spective,  knows  that  the  representation  of  an  object 
changes  as  the  view  point  changes,  so  that  the  picture  of  a 
person  ten  yards  away  will  not  be  the  same  as  a  picture  of 

292 


On  Size  and  Scale 

him  at  a  distance  of  five  yards,  even  if  the  smaller  picture 
from  the  greater  distance  were  enlarged  to  the  size  of 
the  other.  The  picture  will  be  different  at  every  different 
distance,  and  being  different  presumably  some  will  be 
more  satisfactory  than  others.  What  applies  to  a  person 
applies  to  all  solid  objects,  so  that  it  is  not  desirable  to 
regulate  the  size  of  the  image  by  the  distance  of  the 
camera  from  the  object ;  it  is  preferable,  when  the  distance 
is  within  control,  to  find  the  position  that  will  give  the 
best  picture,  and  to  regulate  the  size  of  the  image  in 
some  other  way. 

This  other  way  is  to  adjust  the  distance  of  the  plate 
from  the  lens,  for  the  farther  the  plate  is  drawn  back 
from  the  lens  the  larger  will  the  image  be.  At  twice  the 
distance  the  image  will  be  twice  as  long  and  twice  as 
high.  But  this  means  the  use  of  another  lens  that  will 
give  good  definition  at  the  required  distance,  because  by 
altering  the  distance  the  image  given  by  the  first  lens 
will  be  put  quite  out  of  focus.  A  professional  view  photo¬ 
grapher  who  knows  his  business  will  carry  a  “battery” 
of  lenses,  a  dozen  perhaps  of  different  focal  lengths,  so 
that  after  having  decided  on  the  best  position,  he  can  get 
the  picture  he  wants  of  the  size  he  desires. 

If  the  image  given  by  the  object  is  far  too  small,  either 
because  the  object  is  distant,  like  a  mountain  peak  or  a 
castle  on  the  other  side  of  a  lake,  or  because  it  is  small, 
like  a  flower,  a  butterfly,  or  a  bird,  we  may  find  that  a  lens 
of  three  or  four  feet  focal  length  or  more  would  be 
necessary  to  get  the  image  the  size  we  want.  Of  course 
the  camera  would  have  to  be  long  enough  to  hold  the 
lens  at  one  end  and  the  plate  at  the  other,  and  a  camera 
of  this  length  would  be  a  very  awkward  instrument ;  it 
would  be  troublesome  to  manipulate  and  the  ordinary 
tripod  would  not  support  it  securely.  It  would  obviously 
be  possible  to  take  a  smaller  photograph  and  then  make 
an  enlargement  from  it,  but  this  might  be  more  trouble- 

293 


On  Size  and  Scale 

some  and  would  often  yield  a  result  inferior  to  a  direct 
photograph.  The  telephotographic  lens  exactly  meets 
the  difficulty  here  represented.  Its  name  associates  it 
with  distant  or  telescopic  objects  and  so  far  it  is  unfor¬ 
tunate,  because  it  is  just  as  serviceable  for  near  objects.  It 
is  simply  a  11  large  image  ”  lens,  as  the  author  pointed  out 
when  it  was  introduced,  and  it  gives  a  larger  image  than 
an  ordinary  lens  would  with  the  same  length  of  camera. 
Using  only  a  moderate  power  the  length  of  the  camera 
needed  for  the  same  size  of  image  may  often  be  reduced 
to  the  half  of  that  necessary  with  an  ordinary  lens,  and 
with  higher  powers  the  reduction  may  be  very  much 
greater. 

A  telephotographic  lens  consists  of  an  ordinary  photo¬ 
graphic  objective  with  a  duly  corrected  concave  lens 
between  it  and  the  plate.  The  added  lens  may  be  fixed 
with  regard  to  the  other,  the  whole  forming  a  compound 
lens  that  is  used  exactly  like  any  ordinary  objective,  and 
behaves  in  the  same  way  except  that  the  image  is  larger 
than  the  length  of  the  camera  would  otherwise  permit. 
But  the  magnifying  power  of  the  added  lens  depends  upon 
its  distance  from  the  lens  with  which  it  is  associated,  so 
that  it  is  generally  most  convenient,  and  it  is  usual,  to  have 
this  adjustable. 

Since  telephotographic  lenses  were  first  put  upon  the 
market  by  the  firm  of  Dallmeyer,  they  have  become  a 
necessary  part  of  the  equipment  of  photographers  in 
general  practice,  but  they  are  of  especial  use  to  those  who 
are  interested  in  certain  kinds  of  work.  The  general 
appearance  and  outline  of  a  mountain  peak  which  is 
characteristic  from  a  distance,  will  often  become  so  altered 
on  approaching  it  that  it  not  only  loses  its  grandeur,  but 
it  may  actually  become  unrecognisable  if  not  lost  to  view 
by  the  insignificant  intervening  rocks  coming  into  undue 
prominence.  It  may  be  that  the  only  position  from  which 
one  can  get  a  due  sense  of  the  proportion  of  the  peak  is 

294 


/.  H.  Dallmeyer,  Ltd. 


J.  H.  Dallmeyer ,  I^td. 
The  Clock  Tower,  Westminster 

These  views  show  the  effect  of  a  telephotographic  lens.  The  lower  photograph 
was  taken  under  ordinary  conditions.  The  upper  picture  was  obtained  without 
shifting  the  camera  or  even  extending  it,  but  by  replacing  the  ordinary  lens  by  a 
telephotographic  lens— the  “  Adon.”  Although  the  enlargement  is  so  consider¬ 
able,  and  the  light  available  is  consequently  spread  over  so  much  larger  an  area, 
the  telephotographic  lens  gave  a  sufficiently  brilliant  image  to  render  it  Possible 
to  keep  the  duration  of  the  exposure  short  enough  to  avoid  any  blurring  ot  ttie 
representation  of  the  moving  traffic. 


On  Size  and  Scale 

from  a  distance  of  many  miles,  but  an  ordinary  photo¬ 
graph  taken  from  this  position  would  include  a  wide 
stretch  of  country  with  the  only  interesting  feature  in  the 
picture  so  mixed  up  with  other  details  that  it  would  need 
to  be  marked  for  the  purpose  of  identification.  From  the 
same  point  of  view  the  telephotographic  lens  would  give 
a  picture  larger  to  any  desired  extent,  so  that  either  the 
peak  alone  would  be  shown,  or  just  so  much  of  the  range 
of  mountains  might  be  included  as  the  photographer  con¬ 
sidered  desirable.  Then  there  would  be  no  more  need  to 
indicate  which  was  the  peak  than  there  is  to  mark  what 
represents  the  man  in  a  portrait  of  him. 

A  cathedral  as  a  whole  may  be  photographed  either 
outside  or  inside,  and  many  of  its  parts,  such  as  doors, 
windows,  and  ceilings  may  be  portrayed  without  using 
any  other  than  the  usual  type  of  photographic  apparatus. 
But  if  the  chief  interest  lies  in  the  gargoyles,  or  in  the 
carvings  of  the  capitals,  it  is  very  likely  to  be  impossible 
to  get  sufficiently  near  to  them  to  get  them  on  a  large 
enough  scale  to  show  their  details.  The  telephotographic 
lens  will  give  a  large  and  well-defined  image,  which  will 
have  the  additional  advantage  of  representing  the  detail  as 
it  appears  from  the  position  that  it  is  seen,  and  meant  to 
be  seen  from. 

Animals  in  their  natural  haunts  would  often  be  dis¬ 
turbed  if  the  photographer  approached  them  in  order  to 
photograph  them  as  a  domestic  dog  or  cat  might  be  photo¬ 
graphed.  The  telephotographic  lens  enables  him  to  keep 
at  a  much  greater  distance  without  getting  a  uselessly 
small  picture.  All  sorts  of  devices  have  been  arranged, 
and  many  apparently  insurmountable  difficulties  have  been 
boldly  faced  in  order  to  get  true  pictures  of  birds  and 
animals  as  they  really  are  in  nature.  Sometimes  the 
photographer  will  conceal  himself  in  a  small  erection  ma  e 
to  imitate  the  surrounding  bushes  and  foliage,  so  that  the 
bird's  suspicions  may  not  be  aroused.  He  may  have  to 

295 


On  Size  and  Scale 

take  up  his  position  hours  before  it  is  expected  to  return 
to  its  nest,  for  fear  of  frightening  it  away.  Some  birds 
build  only  in  high  trees,  others  on  the  face  of  cliffs,  and 
seeing  the  photographs  that  have  been  published  by  the 
brothers  Kearton  we  cannot  say  that  any  such  places  are 
inaccessible,  but  it  is  obvious  that  it  must  be  an  advantage 
to  have  the  restrictions  as  to  the  distance  of  the  camera 
made  less  irksome. 

It  is  rarely  if  ever  that  a  gain  can  be  realised  without 
some  loss  that  modifies  the  advantage.  The  chief  difficulty 
with  regard  to  the  use  of  telephotographic  lenses  is  the 
protracted  exposure  necessary.  If  the  image  that  the  lens 
would  give  without  the  added  lens  which  converts  it  into 
a  telephotographic  lens  is  magnified  two  diameters,  the 
light  available  to  form  the  image  is  spread  over  four  times 
the  area— it  is  of  only  one-fourth  the  brightness ;  the  ex¬ 
posure  must  therefore  be  four  times  as  long.  By  the  same 
reasoning  a  magnification  of  eight  diameters  requires  an 
increase  of  the  exposure  to  sixty-four  times  its  original 
duration,  which  is  practically  a  minute  for  every  second. 
A  magnification  of  eight  diameters  would  increase  the 
size  of  a  postage  stamp  to  about  eight  inches  by  six,  rather 
larger  than  the  page  of  this  book,  or  a  bird  that  was  repre¬ 
sented  by  an  image  about  the  size  of  the  end  of  a  lead 
pencil  would  at  this  rate  be  enlarged  to  the  size  of  a  five- 
shilling  piece.  The  only  way  to  reduce  this  increase  of 
exposure  is  to  use  a  lens  of  large  diameter  to  begin  with, 
and  this  is  the  reason  why  telephotographic  lenses  intended 
for  natural  history  work  are  so  large  and  heavy. 

If  the  object  that  we  have  to  deal  with  is  under 
perfect  control — suppose  for  example  that  it  is  a  postage 
stamp — the  camera  with  its  lens  may  be  brought  nearer 
and  nearer  to  it,  and  the  image  if  sharply  focussed  will 
then  become  larger  and  larger.  At  a  certain  stage 
the  image  and  the  object  will  be  of  the  same  size,  and 
that  will  be  when  they  are  equally  distant  from  the 

296 


On  Size  and  Scale 

centre  of  the  lens.  With  such  a  lens  as  is  ordinarily 
used  for  plates  four  or  five  inches  long,  the  stamp  will 
now  be  about  a  foot  in  front  of  the  lens,  and  the 
focussing  screen  with  the  sharp  image  on  it  will  be 
about  the  same  distance  behind  it.  If  now  the  lens  is 
brought  three  inches  nearer  to  the  stamp,  the  image 
will  be  formed  six  inches  farther  away  on  the  other 
side  of  the  lens,  and  it  will  be  twice  as  long  and  twice 
as  wide  as  the  stamp  itself.  Reduce  the  distance  be¬ 
tween  the  lens  and  the  stamp  by  another  inch  and  the 
image  will  be  another  six  inches  farther  away  on  the 
other  side,  and  it  will  show  the  stamp  enlarged  three 
diameters.  The  relative  linear  measurements  of  the 
object  and  the  image  will  always  be  proportional  to 
their  distances  from  the  lens.  By  this  method  we  can 
control  the  size  of  the  photograph  of  the  stamp  until 
the  image  is  produced  so  far  behind  the  lens  that  the 
full  extension  of  the  camera  has  been  brought  into  use. 
It  is  exactly  the  same  method  referred  to  above  by 
which  the  size  of  a  portrait  is  regulated,  but  extended 
until  the  size  of  the  image  exceeds  that  of  the  object. 

Clearly  such  a  method  as  just  described  is  applicable 
only  in  the  case  of  small  objects,  because  the  size  of  the 
photograph  is  limited  by  the  size  of  the  plate.  And  as 
for  general  purposes  of  illustration  it  is  never  desirable  to 
have  the  picture  larger  than  the  object,  this  method  finds 
its  chief  application  in  scientific  work.  The  structure  of 
flowers,  insects,  minerals,  and  similar  things  is  often 
rendered  more  clear  by  a  little  magnification,  and  just 
as  those  interested  in  these  matters  generally  carry  a 
pocket  lens  to  assist  their  eyes,  so  the  photographer 
finds  advantage  in  having  his  picture  somewhat  larger 
than  the  original.  But  this  method  of  increasing  the 
size  of  the  image  soon  reaches  its  limits  because  of  the 
restricted  length  of  ordinary  cameras.  To  give  an  in¬ 
creased  range  cameras  are  made  with  an  extra  long 

297 


On  Size  and  Scale 


extension,  giving  up  portability  for  the  sake  of  a  greater 
range.  But  here  too  the  practical  limit  is  soon  reached, 
for  a  camera  five  feet  long  will  give  a  magnification  of 
only  nine  diameters  when  using  a  lens  of  six  inches 
focal  length.  By  substituting  a  lens  of  three  inches 
focal  length  the  magnification  would  be  doubled,  and 
a  one-inch  lens  would  give  rather  more  than  six  times 
as  much,  practically  a  magnification  of  sixty  diameters. 
But  we  have  now  passed  to  optical  conditions  which  are 
different  from  those  that  the  maker  of  ordinary  photo¬ 
graphic  lenses  seeks  to  satisfy  and  entered  upon  micro¬ 
scopical  work.  Here  we  must  endeavour  to  be  able  to 
push  our  magnifications  to  the  utmost  extent  that  will 
add  to  our  information,  and  although  in  a  sense  photo¬ 
micrography  is  only  an  extension  of  the  use  of  the 
ordinary  camera  and  lens,  the  instrumental  needs  in 
this  field  of  work  are  so  different  that  it  is  convenient  to 
consider  it  as  a  separate  section  of  photographic  work. 

We  have  seen  that  in  increasing  the  size  of  the 
image  given  by  an  ordinary  photographic  objective  we 
reach  a  practical  limit  at  a  magnification  of  some  four  or 
five  diameters.  With  a  microscope  working  under  the 
more  usual  conditions,  if  we  decrease  the  magnification  we 
reach  a  practical  limit  at  about  twenty  diameters.  These 
are  practical  limits  ;  they  can  both  be  exceeded  until  they 
meet  and  overlap,  but  we  then  suffer  the  disadvantage  of 
using  apparatus  under  conditions  that  it  is  not  constructed 
for,  with  the  consequence  that  the  work  done  will  be  in¬ 
ferior  in  quality.  This  difficulty  of  getting  good  results 
with  magnifications  from  about  five  to  twenty  diameters, 
is  now  quite  overcome  by  the  introduction  of  special  | 


lenses,  which  can  hardly  be  called  microscope  objectives! 
because  they  are  not  suitable  for  use  with  eyepieces, 


though  they  are  generally  used  in  conjunction  with  a 

298 


On  Size  and  Scale 

microscope  stand  because  of  the  convenience  that  this 
affords  for  their  manipulation. 

Photomicrography,  as  its  name  implies,  is  the  applica¬ 
tion  of  photography  to  microscopical  purposes.  It  is 
probably  the  first  use  that  photography  was  put  to,  for 
Thomas  Wedgwood  photographed  the  images  given  by  a 
solar  microscope  more  than  a  hundred  years  ago,  and 
some  forty  years  after  this  Herschel  and  Reade  worked 
in  the  same  direction.  The  solar  microscope  was  an 
ordinary  microscope  through  which  direct  sunshine  was 
reflected,  and  the  image  produced  was  received  upon  a 
sheet,  after  the  manner  of  the  image  produced  by  an 
optical  (or  “  magic  ”)  lantern.  But  all  these  experiments 
were  very  crude,  for  the  microscope  then  was  little  more 
than  a  toy  when  compared  with  the  perfected  instru¬ 
ment  as  we  know  it,  and  photography  was  in  its  earliest 
stages. 

We  have  already  seen  that  the  ordinary  photographic 
lens  differs  from  the  microscope  objective.  The  first  is 
intended  to  give  a  comparatively  small  image  of  a  distant 
object,  or  if  the  object  is  nearer  an  image  that  may  equal 
it  in  size  or  is  at  the  most  a  little  larger  than  the  object. 
The  microscope  objective  deals  with  small  objects  which 
are  brought  very  near  to  it,  and  has  to  give  a  large  image. 
But  this  is  not  the  only  difference,  for  the  image  in  the 
camera  is  taken  just  as  the  lens  produces  it,  while  the 
image  in  a  microscope  must  be  so  perfect  that  it  will 
stand  magnification  by  the  eyepiece  and  still  appear  well 
defined.  In  the  construction  of  an  ordinary  photographic 
lens  the  comparative  perfection  of  the  image  is  obtained 
by  correcting  its  aberrations,  the  nature  of  which  we 
have  already  discussed  ;  but  in  a  microscope  objective  this 
is  not  sufficient,  for  we  are  here  getting  towards  the 
absolute  power  of  the  lens,  the  defining  power,  that  is, 

299 


On  Size  and  Scale 

of  any  lens  under  the  given  conditions,  assuming  it  to  be 
entirely  free  from  all  aberrations. 

The  power  of  a  lens  to  give  an  image  showing  minute 
detail  is  called  resolving  power,  because  it  is  expressed 
by  the  minimum  distance  apart  of  two  points  which  the 
lens  is  able  to  show  as  two  separate  points.  Two  points 
closer  than  this  would  be  shown  unresolved  or  unsepa¬ 
rated  and  would  be  indistinguishable  from  a  single  point. 

It  might  be  thought  that  this  power  would  depend  simply 
upon  magnifying  power,  but  that  is  not  so.  Simple  mag¬ 
nifying  power  concerns  the  eye  and  its  ability  to  see 
that  the  two  points  are  separated  in  the  image,  rather 
than  the  actual  fact  of  their  separation  by  the  lens  that 
produces  the  image.  Whatever  eyepiece  is  used  in  the 
microscope  the  resolving  power  of  the  objective  remains 
unchanged.  The  eyepiece  only  magnifies  the  image  pro¬ 
duced  by  the  objective  and  enables  one  to  see  it  better. 
Some  people  who  are  blessed  with  exceptionally  good 
eyesight  will  require  only  a  small  magnification  of  the 
image  to  enable  them  to  see  all  that  there  is  to  be  seen, 
while  others  will  require  a  greater  degree  of  magnification 
to  make  up  for  their  want  of  keenness  of  vision.  As  the 
resolving  power  of  an  objective  is  a  fixed  quantity  and 
does  not  depend  upon  its  magnifying  power,  the  question 
is,  upon  what  does  it  depend  ?  Obviously  the  aberrations 
or  faults  of  the  lens  may  reduce  its  power  in  this  direction, 
but  if  it  were  possible  to  make  a  faultless  lens,  its  resolving 
power  would  still  be  strictly  limited,  because  even  such 
a  lens  would  not  give  an  absolute  point  in  the  image  to 
correspond  with  a  point  in  the  object.  The  best  that 
it  can  do  is  to  give  a  little  disc  instead  of  a  point.  This 
disc  may  be  so  small  that  it  may  appear  to  our  eyes  i 
to  be  a  mere  point  even  when  magnified  ten,  twenty, 
and  sometimes  thirty  diameters,  but  it  only  wants  a  little 

3°° 


On  Size  and  Scale 

more  magnification  to  show  that  it  really  is  a  disc  and 
not  a  point. 

Thus  every  point  of  the  object  is  represented  by  a 
disc  in  the  image,  and  the  smaller  these  discs  are  the  more 
able  is  the  objective  to  separate  or  resolve  fine  detail. 
Why  an  ideally  perfect  lens  would  always  spread  out  or 
thicken  in  the  image  the  detail  of  the  object  we  cannot 
explain  here,  because  the  discussion  of  this  matter  would 
lead  us  too  far  away  from  our  subject,  but  we  must  know 
upon  what  it  depends.  It  depends  upon  the  light  gather¬ 
ing  power  of  the  lens :  not  the  brilliancy  of  the  light,  but 
the  power  of  the  lens  to  gather  what  light  there  is.  We 
may  suppose  that  every  little  point  of  an  illuminated  or 
shining  object  gives  out  light  in  all  directions,  or  if  on  a 
flat  surface,  in  all  directions  into  the  space  bounded  by 
the  surface.  The  greater  the  angle  of  this  light  that  the 
lens  can  receive  and  transmit  to  form  the  image,  the 
smaller  will  be  the  spreading  or  thickening  of  the  details 
represented,  and  the  higher  will  be  the  resolving  power  of 
the  objective. 

The  limit  to  resolving  power  beyond  which  with  our 
present  means  we  cannot  go,  sets  a  practical  limit  to 
useful  magnification.  If  we  can  see  clearly  all  that  there 
is  in  the  image  that  the  lens  produces,  further  magnifica¬ 
tion  can  show  us  no  more  and  is  obviously  useless.  There 
is  no  difficulty  whatever  in  getting  further  magnification, 
and  if  the  photomicrograph  is  to  be  put  up  on  a  wall 
and  examined  by  a  class  at  a  considerable  distance,  it 
may  be  well  to  make  it  large,  but  that  is  a  simple  case 
of  enlarging  and  not  a  matter  of  photomicrography. 
With  a  magnification  of  from  one  to  two  thousand 
diameters,  we  have  reached  the  limit  of  present-day 
microscopical  work,  because  an  average  human  eye  can 
then  see  all  that  there  is  in  the  best  defined  image  that 

3QI 


On  Size  and  Scale 

it  is  possible  to  produce.  This  means  that,  if  there  were 
points  or  lines  side  by  side  so  close  that  it  would  require 
100,000  to  150,000  (using  round  figures)  of  them  to  fill  a 
space  an  inch  long,  they  would  be  shown  as  separate 
points  or  lines.  It  is  not  easy  to  imagine  what  100,000  to 
the  inch  really  means.  If  you  were  to  make  a  straight 
row  of  halfpennies  each  touching  the  one  next  it,  and  were 
to  continue  making  this  row  until  it  was  two  miles  long,  you 
would  then  have  rather  more  than  125,000  halfpennies. 

If  now  you  can  imagine  these  halfpennies  to  shrink,  every 
one  equally  and  still  remain  touching  each  other,  the  row 
would  get  shorter.  When  the  coins  had  shrunk  so  much 
that  the  whole  of  the  two  miles  of  them  had  shrunk  into 
the  distance  occupied  by  only  one  halfpenny,  they  would 
then  be  so  close  that  our  utmost  microscope  power  would 
only  just  be  able  to  show  them  to  us  as  separate  things. 

We  have  given  in  round  figures  the  inferior  and  the 
superior  limits  of  ordinary  microscopy,  but  the  great  bulk 
of  the  subjects  that  need  investigation  by  microscopical 
methods  are  best  treated  by  intermediate  magnifications. 
We  need  no  magnification  in  order  to  count  the  legs  of  a 
house-fly,  but  if  we  wish  to  see  the  minute  hairs  on  its 
tongue  we  need  a  magnification  of  two  to  three  hundred  ^ 
diameters.  We  can  magnify  them  more  than  this,  but 
nothing  will  be  gained  by  doing  so,  for  they  will  merely 
appear  larger.  Indeed  there  is  a  positive  drawback  to 
useless  magnification  in  many  cases  where  the  object  is  , 
not  quite  flat,  because  if  other  matters  are  duly  propor- 1 
tioned,  those  parts  of  the  object  that  are  not  exactly  in  | 
focus  will  be  less  clearly  defined  with  the  greater  than 
with  the  less  magnification. 

In  photomicrography  what  is  commonly  called  a 
microscope  is  used,  more  correctly  a  microscope  stand. 
The  stand  carries  the  objective,  and  if  the  eyepiece  is 

3°2 


Arthur  E.  Smith 


Photomicrograph  of  Teeth  and  Mouth 
of  Medicinal  Leech 


A  rthur  E.  Smith. 


Photomicrograph  of  Palate  of  Limpet 


On  Size  and  Scale 

replaced  by  an  arrangement  to  carry  the  plate,  then  the 
apparatus  becomes  the  equivalent  of  an  ordinary  camera, 
that  is  a  dark  chamber  with  the  lens  that  gives  the  image 
at  one  end  and  the  plate  to  receive  it  at  the  other.  The 
stand,  however,  has  to  carry  the  object  as  well,  for  the 
distance  between  it  and  the  objective  is  often  so  small 
that  the  very  nicest  adjustment  is  necessary  in  regulating 
this ;  and  it  would  be  hopeless  to  attempt  to  do  this 
focussing  unless  they  were  both  supported  rigidly  on  the 
same  stand  with  suitable  power  of  adjustment.  But  so 
far  we  have  only  imitated  the  ordinary  camera.  The 
essential  difference  between  ordinary  camera  work  and 
photomicrography  does  not  consist  in  the  simple  matter 
of  enlargement,  but  in  the  arrangement  of  the  light  that 
falls  upon  the  subject.  The  majority  of  the  objects  as 
prepared  for  microscopical  examination  are  transparent 
and  the  light  is  allowed  to  pass  through  them,  and  as 
light  passing  through  a  stained  glass  window  enables  the 
observer  within  the  building  to  see  the  design,  so  the  light 
renders  visible  or  photographable  the  object  on  the  stage 
of  the  microscope.  But  the  regulation  of  this  light  has 
a  great  deal  to  do  with  determining  what  is  visible  of  the 
object  and  whether  what  is  seen  or  photographed  truly 
represents  the  thing  itself.  It  must  suffice  here  to  say 
that  the  light  that  passes  through  the  object  is  regulated 
with  the  utmost  care  by  systems  of  lenses  that  are  often 
made  with  almost  as  much  precision  as  the  objectives 
themselves.  In  spite  of  all  this  care  and  years  of  study 
there  are  some  well-known  objects  the  characters  of  which 
remain  unsettled,  because  by  altering  the  optical  arrange¬ 
ments  just  a  little  the  image  produced  changes,  and  it  is 
a  matter  of  opinion  as  to  which  of  several  images  is  the 
one  that  shows  the  exact  nature  of  the  object.  This 
applies  chiefly  to  high  power  work,  and  one  example  will 

303 


On  Size  and  Scale 

show  the  kind  of  difference  of  opinion  that  may  exist. 
The  podura  is  a  small  insect  found  in  damp  cellars, 
and  minute  scales  may  easily  be  detached  from  it.  These 
scales  show  markings,  and  it  is  a  matter  of  opinion 
whether  these  markings  are  like  exclamation  marks  (!) 
without  the  dot  at  the  bottom,  like  French  nails  or  pins, 
or  triangular  like  cuneiform  symbols.  The  same  markings 
can  be  photographed  so  as  to  appear  like  either  of  these, 
and  in  each  case  the  image  will  appear  equally  well 
defined. 

The  difficulty  with  regard  to  exceedingly  small  details 
arises  from  the  fact  that  their  size  is  comparable  to 
the  length  of  a  light  wave,  and  when  such  small  things 
interfere  with  the  progress  of  waves,  the  effect  is  not  quite 
the  same  as  the  effect  of  larger  bodies,  as  indeed  we  might 
expect.  If  it  were  possible  to  have  a  row  of  opaque 
particles  getting  regularly  smaller  and  smaller  when  pass¬ 
ing  along  the  row,  it  would  be  found,  under  any  given 
microscopical  conditions,  that  the  image  of  each  particle 
that  we  could  either  see  or  photograph  would  get  smaller 
as  the  particles  were  smaller,  the  size  of  the  image  corre¬ 
sponding  to  the  size  of  the  particle,  down  to  a  certain 
particle.  Beyond  this  the  image  of  each  would  be  of  the 
same  size  but  gradually  become  less  black  as  the  particles 
were  smaller,  until  the  image  faded  into  invisibility,  and 
the  still  smaller  particles  would  not  reveal  their  presence 
in  any  way.  By  using  an  objective  of  the  greatest  light¬ 
gathering  power,  we  could  find  the  smallest  particle  that 
the  microscope  can  show  the  shape  and  size  of,  or,  in 
ordinary  language,  can  be  seen,  and  all  the  particles 
smaller  than  this  would  be  termed  “  ultramicroscopic." 
If  now  the  light  that  shines  into  the  microscope  were 
removed,  and  the  particles  were  illuminated  by  a  light 
that  shines  upon  them  but  not  into  the  instrument  itself, 

3°4 


On  Size  and  Scale 

we  should  get  a  similar  result  to  that  described  above  but 
the  particles  would  appear  bright  on  a  dark  ground.  It 
would  be  possible  now  to  use  an  extremely  bright  light 
and  to  concentrate  it  upon  the  particles,  because  there 
would  be  no  fear  of  hurting  the  eye  or  spoiling  the  photo¬ 
graphic  plate  by  an  excess  of  light,  for  the  only  light  that 
passes  to  the  eyepiece  is  that  small  amount  that  the 
particles  themselves  reflect.  There  would  be  the  same 
uniformity  of  size  of  the  discs  of  light  that  represent  the 
ultramicroscopic  particles  as  before,  but  particles  too 
small  to  be  visible  under  the  previous  conditions  can  now 
be  recognised,  and  the  brighter  the  light  the  smaller  is 
the  particle  that  can  be  seen  by  its  shining.  This  is  the 
principle  of  “  ultramicroscopy,”  a  method  of  work  that 
has  become  of  great  importance  during  the  last  few  years. 
It  is  important  to  remember  that  ultramicroscopy  cannot 
reveal  the  shape  of  a  small  particle,  it  only  shows  the 
presence  of  it  as  a  point  or,  if  much  magnified,  a  disc  of 
light. 

The  methods  of  “dark  ground  "  illumination  are  useful 
for  larger  objects  than  the  particles  that  we  have  just  been 
discussing,  in  fact  for  magnifications  down  to  the  smallest, 
and  are  of  special  use  for  objects  that  are  very  transparent, 
so  that  they,  or  parts  of  them,  are  scarcely  visible  when 
the  light  is  allowed  to  pass  through  them  into  the  micro¬ 
scope  in  the  more  usual  way. 

The  object  of  photomicrography  is  to  represent  the 
nature  and  the  structure  of  the  things  that  it  deals  with. 
In  biological  work  it  is  customary  to  apply  stains  to  the 
preparations,  because  by  their  means  some  parts  will  be 
coloured,  while  other  parts  which  are  of  a  different  char¬ 
acter  will  not  become  coloured  or  will  be  coloured 
differently,  and  thus  the  structure  of  the  object  is  rendered 
more  obvious.  Some  objects,  such  as  minute  organisms, 

305  u 


On  Size  and  Scale 

are  so  transparent  that  they  are  almost  invisible,  and  they 
are  stained  to  make  them  more  easily  seen.  Sometimes 
it  would  be  well  if  the  stained  parts  were  more  deeply 
coloured,  and  perhaps  sometimes  the  colour  is  so  dark 
that  it  hides  the  detail  in  the  parts  that  it  is  desired  to 
emphasise.  In  photomicrography  any  colour  can  be 
made  to  appear  lighter  or  darker  at  will,  and  it  is  thus 
possible  to  produce  a  photograph  that  is  better  than  the 
original  for  the  purpose  for  which  the  preparation  was 
made.  If,  for  example,  the  stain  is  blue,  it  may  be  made 
to  appear  black  by  using  a  deep  yellow,  orange,  or  red 
light,  for  blue  is  blue  because  of  its  power  to  absorb  these 
colours  from  white  light,  and  if  light  of  only  these  colours 
is  used,  the  blue  absorbs  all  the  light  that  there  is,  and  the 
effect  is  the  same  as  if  the  light  were  white  and  the  stain 
black.  But  if  blue  light  is  used,  then  the  blue  stain  will 
allow  this  to  pass  as  readily  as  the  colourless  parts,  and  the 
effect  of  the  stain  is  done  away  with.  By  such  methods 
as  these  the  effect  of  the  colour  may  be  controlled  to  any 
desired  extent. 

But  there  is  another  and  quite  distinct  effect  of  colour. 
We  have  seen  that  there  is  a  certain  minimum  size  of 
particle  that  is  visible  or  photographable  under  any  fixed 
set  of  conditions.  We  have  seen  also  that  this  depends 
upon  the  light-gathering  power  of  the  objective,  and  we 
add  now  that  it  depends  also  upon  the  wave  length  of  the 
light  used  ;  the  shorter  the  wave  the  smaller  the  particle 
that  any  given  objective  can  give  an  image  of.  Here, 
therefore,  after  having  got  the  most  powerful  objective 
from  the  light-gathering  aspect,  we  have  a  means  of 
pushing  our  results  further  into  the  realm  of  the  ex¬ 
cessively  minute.  The  wave  length  decreases  as  we  pass 
from  red  to  green  and  from  green  to  blue  and  violet,  and 
goes  on  decreasing  as  we  pass  into  and  further  into  the 


On  Size  and  Scale 

ultra-violet,  Blue  to  the  eye  is  dark,  violet  very  dark,  and 
the  ultra-violet  gives  no  visible  effect,  so  that  in  this 
direction  we  are  soon  stopped  in  our  visual  observations. 
But  the  photographic  plate  is  strongly  affected  by  the 
violet  and  ultra-violet,  and  by  its  means,  therefore,  we  can 
use  light  of  far  shorter  wave  lengths  than  our  eye  can 
recognise,  and  though  the  image  so  obtained,  with  its 
much  finer  detail,  is  invisible,  it  can  be  photographed,  and 
in  this  sense  we  can  photograph  what  we  cannot  see  and 
can  never  hope  to  see.  By  the  means  at  present  available  in 
this  direction,  it  is  possible  to  get  a  photographic  image  of 
a  particle  that  is  little  more  than  half  the  diameter  of  the 
smallest  particle  that  can  be  seen  by  ordinary  light,  and 
this  means  a  corresponding  increase  in  the  separating  or 
detail-giving  power  in  the  case  of  ordinary  objects. 

We  have  so  far  spoken  of  transparent  or  partially  trans¬ 
parent  objects,  but  opaque  objects  are  also  subject  to  investi¬ 
gation  by  photomicrographic  methods,  and  the  difference 
is  only  a  matter  of  getting  them  suitably  illuminated.  One 
of  the  most  important  of  the  applications  of  this  method  of 
work  is  called  (<  metallography,”  and  it  has  become  almost 
a  separate  art.  The  methods  of  testing  metals  and  alloys 
used  to  be  merely  to  ascertain  the  proportions  of  their 
constituents  and  certain  facts  concerning  them,  such  as  the 
maximum  weight  that  a  wire  of  known  size  could  support, 
their  brittleness,  and  other  such  properties.  Now  it  is  usual 
to  supplement  these  tests  by  polishing  a  surface  of  the 
material,  subjecting  it  to  a  liquid  that  has  a  feeble  solvent 
action  on  it,  and  then  making  a  photomicrograph  of  the 
surface.  The  solvent  or  etching  liquid  never  acts  evenly 
over  the  surface  treated,  because  no  metal  or  alloy  is 
thoroughly  homogeneous.  The  most  readily  attackable 
parts  are  dissolved  away  the  fastest,  and  leave  the  other 
parts  standing  up  in  small  relief.  But  what  is  generally 

307 


On  Size  and  Scale 

of  more  importance  is  that  corrosion  is  usually  accom¬ 
panied  by  a  change  of  colour  of  the  surface  corroded,  and 
this  is  sometimes  of  a  very  striking  character,  as  for 
example  when  pickles  and  fruits  come  in  contact  with  one’s 
dinner  knife.  By  photographing  such  a  prepared  surface 
the  nature  of  the  metal  or  alloy  can  be  ascertained,  whether 
it  is  fibrous,  granular,  or  crystalline,  and  further  whether 
it  is  coarsely  or  finely  granular,  whether  the  crystals  are 
large  or  small,  of  one  or  of  several  kinds,  and  so  on.  This 
knowledge  of  the  form  in  which  the  various  parts  exist, 
that  is,  the  structure  of  the  substance,  is  an  exceedingly 
valuable  guide,  not  only  in  discovering  the  character  of 
the  material  at  hand,  but  as  indicating  in  what  direction 
to  work  in  order  to  prepare  alloys  with  more  useful 
properties. 

But  the  applications  of  microscopical  methods  are  not 
confined  to  laboratory  preparations  or  lifeless  objects. 
The  small  living  organisms  which  are  to  be  found  almost 
everywhere,  in  the  air  as  well  as  in  the  water  and  the 
earth,  can  be  photographed  by  means  of  the  microscope. 
Many  of  them  are  in  constant  movement,  and  by  the  use 
of  a  sufficiently  strong  light  these  can  be  photographed 
instantaneously.  Indeed  the  cinematograph  can  be  used 
in  such  cases  in  conjunction  with  the  microscope,  and 
so  the  actual  movements  of  these  minute  creatures  can 
be  permanently  recorded  for  subsequent  demonstration. 
There  is  therefore  no  break  in  the  continuity  of  the  applica¬ 
tion  of  photographic  methods  on  account  of  the  size  of 
the  object  to  be  studied.  The  photographer  can  adapt 
his  apparatus  to  objects  that  vary  from  the  inconceivable 
magnitudes  of  the  heavenly  bodies,  the  landscapes  that  he 
can  walk  over,  and  the  things  that  he  can  handle,  down 
to  things  that  are  so  minute  that  they  are  as  far  beyond 
our  powers  of  comprehension  on  the  one  hand  as  the 


On  Size  and  Scale 

millions  of  miles  that  astronomy  deals  with  are  on  the 
other. 

And  the  power  of  control  exists  not  only  in  the  size  of 
the  object  dealt  with  but  also  in  the  size  of  the  photograph 
itself.  There  is  a  practical  limit  to  the  size  of  photographs 
taken  directly  in  the  camera,  especially  if  glass  plates  are 
used,  though  in  extreme  cases  it  is  surprising  how  large 
the  apparatus  can  be.  It  is  recorded  that  a  negative,  and 
a  carbon  print  on  a  single  sheet  of  paper,  have  been  made 
seven  feet  by  four  and  a  half  feet,  and  those  who  are 
accustomed  to  deal  with  photographs  about  the  size  of 
their  hands  can  get  a  practical  idea  of  this  size  by  com¬ 
paring  it  with  their  dining-room  table.  The  enthusiasm 
that  would  lead  an  amateur  to  make  a  direct  negative  on 
a  glass  plate  five  feet  by  three  feet  is  very  rare  but  not 
unknown.  The  more  usual  way  of  making  very  large 
photographs  is  to  get  a  small  negative  and  from  this 
to  produce  a  large  image  after  the  manner  of  getting 
pictures  on  the  sheet  with  an  optical  lantern.  Such  an 
enlarged  picture  is  allowed  to  fall  upon  a  sheet  of  sensitive 
paper,  which  is  afterwards  developed  by  means  of  suitable 
apparatus  to  support  it.  Among  the  large  prints  made 
many  years  ago,  there  was  one  thirteen  feet  by  seven  feet, 
but  this  was  made  in  three  sections.  More  recently  a 
bromide  print  was  made  nearly  forty  feet  long  and  five 
feet  wide  on  a  single  sheet  of  paper,  by  exposing  it  in  six 
sections  to  the  enlarged  images  from  six  separate  negatives, 
but  the  development  of  this  huge  print  was  done  in  one 
operation,  the  paper  being  supported  on  a  large  rotating 
drum  made  for  the  purpose. 

It  may  be  of  interest  to  pass  now  to  the  other  extreme 
in  the  sizes  of  photographs.  The  little  views,  about  a 
twentieth  of  an  inch  long,  that  are  sometimes  mounted 
with  a  globule  of  glass  to  magnify  them  in  the  ends  of 

309 


On  Size  and  Scale 

pensticks  and  other  fancy  articles,  are  produced  by  using 
a  microscope  in  a  reverse  direction.  The  illuminated 
object  is  put  in  the  position  usually  taken  by  the  eye 
and  the  sensitive  plate  takes  the  ordinary  place  of  the 
object  on  the  stage  of  the  instrument.  In  this  way  a 
microscopic  image  is  produced,  and  it  is  only  necessary 
to  take  care  that  the  texture  of  the  film  is  not  too  coarse, 
to  get  on  development  and  fixing  a  tiny  photograph  of 
fine  enough  detail  to  stand  considerable  magnification. 
This  method  of  “  microphotography "  became  of  very 
great  importance  in  many  ways  during  the  Siege  of  Paris 
in  1870-71.  Pigeons  could  pass  with  fair  regularity 
into  or  out  of  the  city,  as  they  had  been  trained  to  do 
so.  But  a  pigeon  can  only  carry  a  load  of  a  few  grains 
in  weight,  twenty  or  thirty  perhaps  at  the  most.  By 
setting  up  the  suitably  printed  despatches  and  photograph¬ 
ing  them  on  a  very  small  scale  on  thin  collodion  films, 
about  a  thousand  communications  could  be  obtained 
upon  a  single  pellicle  a  little  larger  than  three  postage 
stamps.  By  means  of  a  projection  microscope  and  an 
arc  lamp,  an  enlarged  image  of  the  microscopic  news 
sheet  was  thrown  upon  a  screen,  and  the  messages 
were  copied  and  despatched  as  required.  The  whole 
series  of  despatches  transmitted  during  the  siege  amounted 
to  about  a  hundred  and  fifteen  thousand,  and  as,  in  order 
to  make  sure  of  safe  carriage,  the  communications  were 
duplicated  from  twenty  to  forty  times,  as  many  as  two 
and  a  half  millions  were  photographed  and  transmitted 
during  the  two  months  of  the  investment.  A  single 
series  of  the  one  hundred  and  fifteen  thousand  despatches 
in  the  form  in  which  they  were  transmitted,  weighed 
only  about  thirty  grains.  M.  Dagron,  who  organised 
the  pigeon  post,  used  a  multiple  camera  with  twenty 
lenses,  producing  twenty  photographs  at  once,  in  order 
to  expedite  matters. 

When  we  consider  into  what  an  exceedingly  minute 

310 


On  Size  and  Scale 

space  it  is  possible  to  obtain  so  much  reading  matter, 
we  see  a  possible  solution  of  the  difficulty  of  the  growth 
of  our  national  libraries.  Such  pellicles  would  probably 
be  more  lasting  than  the  paper  used  at  the  present  time 
for  newspapers  and  magazines,  and  it  would  be  easy  to 
arrange  so  that  the  reading  of  the  minute  sheets  was 
no  more  trouble  than  the  looking  at  pictures  in  a  stereo¬ 
scope. 


CHAPTER  XIX 

SUNDRY  APPLICATIONS  OF  PHOTOGRAPHY 

We  have  already  referred  to  many  of  the  uses  of  photo¬ 
graphy,  and  to  some  in  considerable  detail,  though  chiefly 
by  way  of  illustration  or  because  of  the  intimate  connection 
of  the  matter  with  the  subject  under  discussion.  As 
already  indicated,  it  would  be  absurd  to  attempt  to  give 
anything  like  a  complete  survey  of  photography  in  its 
applications,  but  a  few  of  the  more  obvious  of  these, 
and  especially  such  work  as  has  been  rendered  possible 
by  photography  and  would  be  impossible  without  it,  may 
well  claim  attention  in  connection  with  our  subject. 

We  naturally  turn  first  to  portraiture,  for  there  are 
persons  who  still  apply  the  word  “photographer”  only 
to  those  who  get  their  living  by  making  portraits,  and 
it  is  the  portrait  photographer  who  is  commonly  dis¬ 
tinguished  as  a  “professional  photographer.”  Until 
photography  was  available  for  the  purpose,  portraits 
were  a  luxury  for  the  rich,  now  they  are  often  but  little 
valued  by  the  poor,  though  no  one  can  estimate  the 
benefit  to  all  classes  by  the  possibility  of  good  and  cheap 
portraiture.  The  one  interfering  circumstance  in  the 
practice  of  this  branch  of  the  art,  from  a  technical  point 
of  view,  is  the  variability  of  daylight  from  day  to  day 
as  well  as  through  the  changing  seasons,  and  it  was  only 
natural  that  those  whose  livelihood  depended  upon  this 
work  should  desire  some  method  of  replacing  daylight 
by  a  more  constant  source  of  illumination.  The  first 
really  successful  effort  in  this  direction  was  by  Mr.  Henry 

312 


Sundry  Applications  of  Photography 

Van  der  Weyde,  who  fitted  up  a  studio  in  Regent  Street, 
London,  from  which  daylight  was  excluded.  Although 
electric  lighting  was  at  that  time  (1877)  only  in  its  early  in¬ 
fancy,  he  installed  a  gas-engine  and  a  dynamo,  and  devised 
an  electric  arc  lamp  that  was  so  satisfactory  that  it  is  still 
in  use.  The  light  from  the  arc  did  not  shine  directly 
upon  the  sitter,  there  was  a  small  screen  to  prevent  that, 
it  illuminated  the  inside  of  a  large  umbrella-like  reflector 
that  served  to  moderate  its  intensity  and  diffuse  it  more 
pleasantly. .  The  whole  arrangement  was  mounted  so  that 
it  could  be  easily  brought  into  any  desired  position. 

Since  then  many  kinds  of  lamps  have  been  constructed 
for  portraiture,  and  those  giving  a  light  of  less  intensity 
than  the  arc,  such  as  incandescent  electric,  incandescent 
gas,  acetylene,  &c.,  are  generally  arranged  in  groups  in 
order  that  sufficient  light  may  be  obtained  to  keep  the  ex¬ 
posure  necessary  down  to  a  second  or  two.  Very  few 
portrait  photographers  have  followed  Mr.  Van  der  Weyde 
in  excluding  daylight  altogether,  chiefly  perhaps  because 
of  the  expense  ;  it  is  more  customary  to  use  artificial  light 
only  when  the  daylight  fails.  But  in  trade  works,  that  is 
where  all  branches  of  photography  are  carried  on  except 
portraiture  and  scientific  work,  it  is  quite  usual  now  to  use 
only  artificial  light.  This  obviates  all  difficulties  with 
regard  to  the  windows  of  the  room  or  studio  ;  the  light 
being  always  the  same  the  exposures  can  be  timed  to  a 
nicety,  and  summer  or  winter,  day  or  night,  the  work  can 
be  carried  on  with  equal  facility. 

The  portraiture  of  criminals  as  systematically  done  in 
prisons  is  not  so  important  a  matter  now  as  it  used  to  be, 
because  the  identity  of  prisoners  is  generally  registered  or 
established  by  means  of  “finger  prints.”  The  ridges  of  the 
skin  at  the  ends  of  the  fingers  are  constant  through  life. 
They  may  be  obliterated  by  accident  but  they  cannot  be 
altered  ;  in  this  they  differ  from  the  outline  and  general 
appearance  of  the  face,  and  are  therefore  to  be  preferred 

3*3 


Sundry  Applications  of  Photography 

for  this  purpose.  The  getting  of  finger  prints  has  no 
connection  with  photography,  the  finger  is  merely  pressed 
on  an  inked  surface  and  then  on  paper  to  get  the  impres¬ 
sion,  but  it  is  often  desirable  to  make  a  photographic 
enlargement  of  the  print  for  comparing  it  with  others, 
particularly  the  prints  made  accidentally  by  criminals 
during  their  handling  of  such  things  as  window-panes, 
metal  boxes,  or  anything  with  a  surface  that  is  susceptible 
to  being  marked  by  dirty,  greasy,  or  perspiring  hands. 

A  portrait  is  in  essence  nothing  more  than  a  record 
of  the  appearance  of  an  individual,  whether  it  serves  to 
identify  a  criminal,  to  recall  the  features  of  a  friend,  or  to 
introduce  notable  men  and  women  to  the  general  public. 
There  are  innumerable  other  interesting  things  besides  the 
faces  of  those  we  know  or  wish  to  know,  and  all  are  liable 
to  change.  How  we  should  be  interested  in  seeing  photo- 
'  graphs  of  London  and  its  characters  and  buildings  at  the 
time  preceding  the  great  fire — the  people  at  work  and  at 
play,  as  they  travelled  and  as  they  lived  at  home  !  Within 
the  last  half  century  there  have  been  vast  alterations  in 
London.  Miles  of  open  country  have  been  turned  into 
paved  streets,  while  many  houses  and  streets  of  historic 
interest  have  totally  disappeared  ;  and  the  same  is  true,  in 
a  greater  or  less  degree,  of  all  towns.  The  coastline  in 
many  parts  is  constantly  changing,  so  that  the  sites  of 
what  were  flourishing  towns  are  now  a  mile  or  more  out 
at  sea,  while  some  old  seaports  are  nearly  as  far  inland. 
There  are  many  who  would  like  to  see  pictures  of  places, 
people,  and  fashions  as  they  were  long  ago,  not  to  satisfy 
an  idle  curiosity,  but  as  helps  in  the  study  of  history  and 
for  scientific  purposes.  Such  pictures,  however,  are  com¬ 
paratively  very  few,  and  some  are  obviously  more  fanciful 
than  real.  We  cannot  go  back  to  remedy  this  deficiency,  | 
but  it  is  possible  to  see  that  our  own  times  are  recorded,  | 
that  those  who  come  after  us  may  not  suffer  as  we  do. 

And  indeed  no  one  can  tell  how  soon  such  pictures  may 

_  T  . 


Sundry  Applications  of  Photography 

become  of  value,  for  sudden  disaster  may  at  any  time  lead 
to  great  changes. 

The  idea  of  starting  and  keeping  up  a  pictorial  record 
of  things  occurred  to  some  archaeological  societies,  and 
doubtless  to  others,  before  the  introduction  of  gelatine 
dry  plates  set  photography  on  a  new  basis.  But  when 
photography  was  popularised  by  this  means,  the  method 
that  seemed  necessary  was  available,  and  there  was 
good  reason  to  hope  that  many  amateurs  would  interest 
themselves  in  such  work.  Mr.  W.  Jerome  Harrison 
of  Birmingham  made  definite  suggestions  in  1885  and 
subsequently,  and  in  1889  he  succeeded  in  getting  the 
Birmingham  Photographic  Society  to  inaugurate  a 
scheme  for  carrying  out  a  “  photographic  survey  of 
Warwickshire."  A  few  other  societies  started  work  of 
this  kind  in  different  parts  of  the  country,  and  progress 
was  being  made  on  a  rather  small  scale  when,  in  1897, 
Sir  J.  B.  Stone  called  a  meeting  to  which  he  invited 
many  persons  likely  to  be  interested,  and  the  National 
Photographic  Record  Association  was  established.  This 
association  worked  for  twelve  years,  not  only  in  collecting 
photographs,  but  in  seeking  to  arouse  interest  in  the 
matter  in  the  many  local  photographic  societies.  In 
1909,  their  success  in  this  direction  led  to  the  disbanding 
of  the  central  society  that  the  work  might  be  carried 
on  from  various  local  centres.  The  National  Photo¬ 
graphic  Record  Association  accumulated  nearly  five 
thousand  prints,  and  these  are  deposited  in  the  British 
Museum.  The  Warwickshire  Society  has  collected  some 
thousands,  and  the  various  other  societies  taken  together 
a  few  more  thousands,  and  all  these  are  deposited  in 
various  museums  and  public  libraries  in  charge  of  the 
local  authorities.  These  photographs  represent  buildings 
of  interest  of  all  kinds,  Roman  and  other  ancient  remains, 
manuscripts,  portraits  of  well-known  people,  ceremonies, 
— such  as  the  coronation,  customs — such  as  the  dis- 

315 


Sundry  Applications  of  Photography 

tribution  of  Maundy  money,  fairs  and  May  Day  cele¬ 
brations,  and  other  matters  that  are  likely  to  be  of 
future  interest.  The  method  of  work  is  to  divide  up  the 

district  among  those  willing  to  assist,  and  if  some  have 

special  knowledge  the  division  may  extend  to  the  subjects 
as  well ;  the  photographs  are  made  of  prescribed  sizes  and 
mounted  on  standard  mounts,  with  all  the  necessary 
details  concerning  the  object  written  on  them.  Only 
permanent  prints  are  accepted,  such  as  those  made  by 
the  platinum  and  carbon  processes.  This  work  is  still 
going  on,  and  there  seems  no  reason  to  doubt,  if  it  is 

persevered  in,  that  the  historians  of  a  few  centuries 

hence  will  have  much  more  reliable  information  as  to 
our  manners  and  customs  and  surroundings  than  we  have 
of  those  that  preceded  us.  By  the  kindness  of  Mr.  George 
Scamell  we  are  able  to  show  an  example  of  a  series  of 
“ street  cries”  made  by  Mr.  Edgar  Scamell,  and  other 
illustrations  are  given  showing  the  nature  of  such  re¬ 
cords. 

There  is  a  good  deal  of  photographic  work  done  of 
a  somewhat  similar  kind,  though  more  technical  in 
character  and  generally  for  more  immediate  use.  There 
are  many  large  estates  at  or  near  the  great  centres  of 
industry,  on  which  are  a  vast  number  of  buildings,  some 
old  and  some  of  exceptional  interest.  In  the  management 
of  such  estates  it  is  constantly  necessary  to  remove 
houses  that  they  may  be  replaced  by  others  better  adapted 
to  the  needs  of  the  present  day,  and  it  is  not  unusual  to 
make  a  rule  of  having  every  building  that  is  to  be  pulled 
down  or  altered,  photographed  before  the  changes  are 
begun.  And  if  the  property  of  another  owner  is  close 
to  where  an  alteration  is  to  be  made,  it  is  desirable 
that  photographic  records  of  adjacent  buildings  be  made, 
for  persons  have  been  known  to  make  dishonest  claims 
as  to  ancient  lights,  damage  done  by  the  disturbance 
of  foundations,  and  so  on.  Even  if  the  suggestion  of 


F.  V.  T.  Lee 


F.  V.  T.  Lee 


Records  of  an  Engineering  Firm 

Copies  of  the  actual  photographs  taken  by  a  firm  of  engineers  as  records  of  work  done 
and  the  condition  of  machinery  needing  repair.  The  upper  photograph  shows  the  corrosion 
that  has  taken  place  in  the  runner  of  a  turbine.  The  lower  photograph  is  one  of  a  series 
taken  day  by  day  to  show  the  progress  of  the  erection  of  a  power  station,  lhis  firm 
photographs  all  their  work  in  this  manner.  The  prints  are  made  on  post  cards,  which 
are  filed,  as  is  customary  with  card  indexes. 


Sundry  Applications  of  Photography 

fraud  is  entirely  dismissed,  it  is  possible  for  old  defects 
to  be  brought  to  light,  defects  of  which  the  owner  had 
no  knowledge  and  which  he  honestly  considers  are  due 
to  the  disturbances  caused  by  the  changes  being  carried 
out.  A  photograph  taken  before  the  work  was  begun 
may  convince  him  of  his  error,  and  so  save  the  cost 
and  the  unpleasantness  of  litigation.  It  has  been  stated 
that  in  making  underground  railways,  it  saves  expense 
to  have  every  building  that  is  likely  to  be  affected 
photographed  before  the  work  is  begun. 

Although  work  of  this  kind  cannot  be  considered  as 
of  a  very  critical  character,  it  is  distinctly  technical  and 
far  removed  from  what  is  known  as  snapshotting.” 
Indeed  a  person  may  be  well  skilled  in  some  branches  of 
photography  and  prove  unable  to  carry  out  such  work 
satisfactorily,  just  as  a  person  may  be  able  to  write  beauti¬ 
fully  and  be  a  master  of  language,  and  utterly  fail  when 
confronted  with  a  technical  subject.  On  the  other  hand, 
a  surveyor  who  thinks  that  he  has  only  to  buy  a  camera 
and  follow  his  instruction  book,  is  comparable  to  a  man 
who  endeavours  to  translate  from  a  language  he  knows 
nothing  of  by  the  aid  of  a  dictionary.  The  photographs, 
like  the  translation,  may  happen  to  be  successful  here  and 
there,  but  they  will  be  almost  valueless,  for  the  successes 
will  be  indistinguishable  from  the  mass  of  uncertainties, 
without  independent  evidence. 

By  taking  photographs  at  suitable  intervals,  and  dating 
and  filing  them,  a  complete  history  of  the  progress  of 
constructional  work  in  a  factory  may  be  obtained  in  a 
permanent  form.  By  the  kindness  of  Mr.  F.  V.  T.  Lee  of 
California  we  are  able  to  give  an  actual  example  of  such 
a  series  taken  during  the  erection  of  a  power  station. 
These  photographs  are  not  intended  to  give  much  detail, 
but  they  show  the  general  progress  of  the  building  in  a 
more  concise,  and  generally  more  complete,  and  obvious 
manner  than  any  short  verbal  descriptions  could  do. 

3J7 


Sundry  Applications  of  Photography 

All  the  work  done  at  this  establishment  is  photographed  at 
suitable  intervals,  prints  are  made  upon  sensitised  post¬ 
cards,  as  this  obviates  the  need  for  mounting  them,  all 
the  necessary  details — description,  date,  time  of  day,  and 
photographic  particulars — are  filled  in  on  a  blank  form  on 
the  back,  and  the  photographs  are  then  put  away  after 
the  manner  of  a  card  index— a  far  superior  method  of 
keeping  such  records  than  pasting  them  in  a  scrap  book, 
as  reference  to  them  is  facilitated,  and  any  one  may  be 
removed  if  desired.  The  pictorial  record  of  work  extends 
also  to  the  condition  of  machinery  received  that  needs 
repair,  and  an  example  of  this  is  given  on  the  same  page 
as  the  power  station,  showing  the  runner  of  a  turbine 
and  the  extensive  corrosion  that  has  taken  place.  If  an 
accident  should  happen,  it  would  be  the  duty  of  those  who 
are  available  to  photograph  the  scene  and  its  details  at 
once,  though  nothing  would  be  allowed  to  stand  in  the  way 
of  rendering  immediate  aid  to  anyone  suffering  from 
injury.  We  are  assured  that  this  complete  system  of 
filing  photographic  records  is  of  inestimable  advantage  in 
engineering  works.  It  is  not  simply  that  it  provides  in¬ 
formation  that  is  of  use  to  those  connected  with  the 
establishment,  but  it  often  saves  disputes  with  all  their 
attendant  troubles  and  expense,  and  if  an  action  at  law  is  ; 
unavoidable  it  provides  valuable  evidence.  By  the  use 
of  suitable  apparatus  and  films  for  negatives  the  time 
occupied  in  the  making  of  these  photographs  is  not  worth 
consideration.  It  is  of  interest  to  note  that  all  the  engineers 
employed  on  this  establishment  are  expected  to  be  able 
to  do  this  photographic  work,  just  as  they  are  expected  ■ 
to  be  able  to  make  such  written  reports  as  may  be 
necessary. 

It  is  but  a  step,  though  a  long  and  important  one, 
from  the  kind  of  photography  that  we  have  been  con-  I 
sidering  to  photogrammetry  or  photographic  surveying, 
that  is  the  art  of  preparing  photographs  that  shall 

3i8 


Sundry  Applications  of  Photography 

serve  the  purpose,  and  from  them  drawing  plans  to 
scale  of  the  country  surveyed.  It  must  not  be  supposed 
that  those  whose  business  it  is  to  draw  maps  are  in  any 
case  pledged  to  depend  entirely  upon  the  camera,  it  is 
only  brought  into  use  when  the  work  can  be  done 
better  by  its  means  than  in  any  other  way.  A  photo¬ 
graphic  survey  therefore  is  one  done  chiefly,  but  not  of 
necessity  entirely,  by  photographic  means.  Photography 
offers  the  advantage  of  giving  the  whole  view  with  its 
details  in  a  moment,  and  this  may  render  possible  what 
would  be  quite  impossible  by  the  more  usual  methods 
of  topograpical  surveying.  M.  E.  Deville  in  a  recent 
report  gives  an  excellent  example  of  this.  In  1892  a 
commission  was  appointed  to  study  some  600  to  700 
miles  of  the  frontier  between  Alaska  and  Canada,  and 
to  map  it,  and  report  within  three  years.  But  this 
district  is  mountainous  and  suffers  from  almost  con¬ 
tinuous  rain  and  mist.  It  is  impossible  to  survey  by 
any  method  when  the  country  to  be  mapped  cannot  be 
seen,  but  by  taking  advantage  of  the  bright  intervals  in 
the  three  short  summer  seasons  available,  3000  photo¬ 
graphs  were  taken  and  a  satisfactory  map  was  made. 
The  total  area  surveyed  photographically  in  Canada  is 
equal  to  about  25,000  square  miles,  or  an  area  rather 
greater  than  Holland  and  Belgium  put  together.  There¬ 
fore,  however  much  improvement  remains  to  be  effected 
in  the  application  of  photography  to  this  purpose,  the 
time  is  past  when  it  is  possible  to  doubt  its  applicability 
and  advantages.  We  are  not  concerned  here  with  enter¬ 
ing  into  the  details  of  the  method  employed.  It  must 
suffice  to  say,  that  the  cameras  used  must  be  specially 
constructed  so  that  they  may  be  accurately  levelled  and 
in  other  ways  strictly  dependable,  and  that  it  is  desirable 
to  have  specially  corrected  lenses  so  that  distortion  may 
be  minimised. 

There  have  been  several  different  cameras  designed 

3i9 


Sundry  Applications  of  Photography 

to  facilitate  surveying.  Some  carry  flat  plates  and  there¬ 
fore  represent  the  view  in  plane  projection.  Others 
have  curved  plates  or  films,  curved  round  the  lens  as  a 
centre,  and  give  the  view  in  cylindric  projection.  Photo¬ 
graphs  of  the  latter  kind  (panoramic)  have  the  advantage 
of  giving  a  much  greater  extent  of  the  view  in  one 
direction  than  is  possible  with  a  flat  plate,  and  for  that 
reason  the  method  is  sometimes  employed  for  other 
purposes  than  surveying.  There  is  a  certain  point  with 
regard  to  every  lens  about  which  it  may  be  rotated  on 
an  axis  parallel  to  its  surface,  without  moving  the  image 
that  it  produces,  and  the  distinguishing  feature  of  a 
panoramic  camera  is  that  the  lens  swings  round  on  this 
axis  as  the  exposure  is  being  made,  so  that  the  sensitive 
surface  is  exposed  gradually,  as  the  lens  swings,  from 


one  end  to  the  other. 

Six  years  after  Daguerre  published  his  process,  F.  v. 
Martens,  a  copperplate  engraver  of  Paris,  made  a  pano¬ 
ramic  or  “traversing”  camera  to  take  curved  Daguerreo¬ 
type  plates.  Fig.  23  is  an  illustration  of  this  camera 
taken  from  the  catalogue  of  George  Knight  and  Sons  of 
that  period.  It  may  be  noted  in  passing  that  the  plates 
for  these  cameras  cost  from  4s.  6d.  to  17s.  each  accord¬ 
ing  to  size.  After  this,  flat  plates  were  arranged  for,  by 
giving  them  a  rolling  movement  against  the  curved 
surface  as  required  for  the  exposure.  When  flexible 
films  were  made  commercially,  several  cameras  of  this 
type  were  devised,  some  for  surveying  purposes,  and 
some  such  as  the  “panoram  kodak”  for  general  use 
by  amateurs  and  others.  This  last  camera  is  of  quite 
simple  construction,  similar  to  Marten’s,  but  the  lens  is 
rotated  by  a  spring  instead  of  by  hand,  and  there  is  a 
chamber  at  each  end  to  hold  the  spool  of  film. 

In  the  method  of  surveying  just  referred  to  the  plate 
is  kept  strictly  vertical,  the  lens  points  in  a  horizontal 
direction,  and  the  distances  of  objects  are  ascertained  by 


320 


Sundry  Applications  of  Photography 

measuring  the  relative  displacement  of  them  in  photo¬ 
graphs  taken  from  two  or  more  standpoints.  But  anyone 
who  has  viewed  the  country  from  an  eminence,  looking 
down  upon  it  from  the  top  of  a  tower  or  a  hill,  knows  that 
the  bird’s-eye  view  so  obtained  is  a  map-like  picture  of 
the  district.  The  houses,  trees,  hedges,  and  roads,  instead 
of  being  behind  each  other  as  when  seen  from  the  level, 


or  a  low  elevation,  are  separated  so  that  they  can  be  dis¬ 
tinctly  identified.  Such  a  view  is  sometimes  exactly  what 
is  wanted  in  military  operations,  but  the  tower  or  the  hill 
is  not  always  at  hand,  or  if  an  eminence  is  available  it  may 
not  be  high  enough.  It  is  here  that  the  balloon  shows  its 
advantages.  It  appears  that  M.  Nadar  was  the  first  to 
photograph  from  a  balloon,  but  the  results  he  obtained  in 
1859  in  connection  with  the  war  between  Italy  and  Austria 
were  not  very  satisfactory.  But  the  matter  was  not  allowed 
to  rest,  and  some  quite  notable  results  were  subsequently 

321  x 


Sundry  Applications  of  Photography 

obtained.  The  introduction  of  dry  plates  greatly  facilitated 
such  work,  and  now  the  military  authorities  of  all  civilised 
countries  recognise  the  importance  of  being  able  to  make 
observations  of  this  character.  Cameras  have  been 
attached  to  kites  and  the  shutter  released  while  in  mid-air 
by  various  contrivances,  but  although  this  method  is  suit¬ 
able  for  some  purposes,  it  does  not  allow  of  the  control 
and  discrimination  possible  when  the  photographer  himself 
goes  up  with  the  instrument.  With  the  advent  of  flying 
machines  we  may  expect  to  see  revolutionary  changes  in 
all  aerial  work,  but  exactly  in  what  direction  these  changes 
will  be  effected  it  is  impossible  to  forecast.  Meanwhile  the 
photograph  of  Stonehenge  (facing  page  188)  taken  fiom  an 
aeroplane  shows  the  possibility  of  such  work  in  spite  of  the 
great  rate  at  which  flying  machines  must  travel. 

It  is  common  enough  to  photograph  the  clouds,  for 
almost  all  out-of-doors  views,  whether  taken  for  scientific  : 
purposes  or  merely  for  amusement,  include  a  part  of  the  I 
sky.  But  it  is  a  well  known  fact  that  the  sky,  including  of 
course  the  clouds,  is  very  rarely  as  clearly  shown  in^  the 
photograph  as  it  appears  to  the  eye.  This  is  due  to  tnree  j 
well  understood  reasons.  If  a  sufficient  exposure  has  been 
given  for  the  other  part  of  the  view,  the  sky  will  have  had 
an  excessive  exposure,  and  over  exposure,  as  we  have  i 
shown  in  earlier  chapters,  causes  a  want  of  contrast  in  the  , 
picture  because  the  brighter  parts  do  not  continue  to  pro¬ 
duce  a  proportionally  increasing  effect  upon  the  plate. 
This  can  be  remedied  by  making  the  exposure  suitable  for 
the  clouds  and  sacrificing  the  rest.  It  will  often  happen  i 
that  blue  sky  and  white  cloud  are  hardly  distinguishable  in  ! 
the  photograph,  because  of  the  excessive  sensitiveness  of  i 
the  plate  to  blue.  That  is,  the  plate  is  affected  similarly 
whether,  as  in  the  white  light  from  the  cloud,  the  blue  is[ 
mixed  with  green  and  red  to  give  the  white,  or  whether,  asij 
in  the  sky,  the  blue  predominates  or  is  alone.  This  is  ► 
remedied  by  increasing  the  sensitiveness  of  the  plate 

322 


Sundry  Applications  of  Photography 

to  green  and  red  and  using  a  yellow  filter  or  screen 
to  absorb  some  of  the  blue  light.  The  third  reason  is, 
that  there  is  always  a  certain  amount  of  mist  in  the  air 
due  to  the  small  particles  in  which  it  abounds  and  which 
can  be  seen  in  a  beam  of  bright  light.  These  particles 
reflect  or  scatter  the  light  according  to  their  size,  the  larger 
particles  scattering  light  of  a  longer  wave  length.  The 
smallest  particles  appear  to  be  always  present,  and  it 
depends  upon  meteorological  conditions  as  to  what  extent 

I  these  are  mixed  with  particles  of  larger  sizes.  Rain  “  clears 
the  air  ”  as  we  say,  washing  down  these  larger  particles, 
and  the  wind  has  a  considerable  effect  in  distributing  them 
as  they  rise  from  fires  or  whatever  smokes  or  fumes  or  stirs 
up  dust.  Small  particles  scatter  ultra-violet  light,  rather 
larger  particles  scatter  blue,  larger  still  the  green  and  then 
the  red.  So  far  as  light  is  scattered  by  aerial  particles,  the 
air  is  misty,  and  as  photographic  plates  are  sensitive  chiefly 
to  the  blue  and  violet  and  ultra-violet,  as  the  particles  in¬ 
crease  in  size  the  air  becomes  misty  to  the  plate  before  it 
is  misty  to  our  eyes,  because  the  brightest  colours  to  us 
are  green  and  yellow.  So  that  the  air  may  be  misty 
photographically  and  clear  visually,  and  it  is  invariably  the 
case  that  it  is  more  misty  to  the  plate  than  to  the  eye.  For 
this  reason  all  objects  at  a  distance,  including  clouds, 
appear  in  photographs  as  if  seen  through  a  mist,  and  a 
denser  mist  than  we  can  see.  This  difficulty  is  remedied 
by  stopping  all  the  ultra-violet  and  a  large  portion  of  the 
blue,  or  it  may  be  all  the  violet  and  blue,  by  fixing  a  suitable 
deep  yellow  filter  to  the  lens.  By  observing  these  pre¬ 
cautions  it  is  possible  to  photograph  clouds  with  all  their 
detail  and  intensity  and  to  produce  pictures  suitable  for 
the  study  of  them,  though  as  they  are  so  constantly  and 
rapidly  changing,  it  is  impossible  to  make  fully  detailed 
drawings  of  them  by  hand. 

The  heights  of  clouds  can  be  measured  in  different 
ways.  Sometimes  they  are  so  low  that  the  upper  parts  of 

323 


/ 


Sundry  Applications  of  Photography 

high  buildings  are  hidden,  or  the  mountaineer  or  the 
aeronaut  may  pass  through  them  on  his  upward  journey, 
and  then  their  height  can  be  measured  by  direct  observa¬ 
tion.  But  there  may  be  no  building  or  mountain  and  no 
balloon  or  flying  machine  at  hand,  and  there  may  be 
clouds  above  the  reach  of  such  means,  so  that  these 
methods  are  not  of  general  applicability.  By  photograph 
ing  them  from  two  points  simultaneously,  and  measuring 
on  the  photographs  their  apparent  alteration  in  position 
due  to  the  different  points  of  view,  their  heights  can  be 
determined  by  ordinary  trigonometrical  methods.,  The 
two  cameras  must  be  so  connected  that  the  shutters  of 
both  are  released  at  the  same  moment,  but  this  arrange¬ 
ment  offers  no  practical  difficulty. 

There  is  nothing  to  be  said  of  a  general  character  as 
to  the  photography  of  rainbows  and  auroras,  and  by 
the  use  of  screen  colour  plates  they  can  be  shown  in 
colour.  Photographs  of  these  phenomena  are  of  interest 
because  the  opportunities  of  getting  them  are  so  rare. 
Lightning  is  also  comparatively  rare,  and  the  method  of 
photographing  its  flashes  is  essentially  different  from  the 
usual  process,  because  they  do  not  remain  for  long  enough 
to  permit  one  to  focus  and  expose,  or  even  to  expose  the 
plate  if  the  focus  has  been  previously  adjusted.  It  is  a 
mistake,  however,  to  suppose  that  lightning  is  an  exceed-  j 
ingly  rapid  effect,  for  a  careful  observer  will  frequently  find  | 
that  after  seeing  the  first  glare,  he  has  time  to  turn  his  , 
eyes  in  its  direction  and  to  actually  see  the  flash  itself.  |j 
This  appears  to  be  due  to  the  multiple  character  of  many 
flashes.  The  first  discharge  is  not  complete,  and  a  second, 
third,  and  even  a  fourth  may  take  place  along  the  same  j 
path,  as  if  the  first  had  made  an  easier  way  for  the  sub-  j 
sequent  discharges. 

Lightning  is  photographed  at  night  by  pointing  the  :| 
camera  towards  the  direction  where  it  is  likely  to  occur,  I 
keeping  the  lens  open.  If  no  flash  comes  and  it  is  con-  | 

324 


Taken  with  a  camera  held  in  the  hand  and  moving  sideways.  The  three  main  flashes  would  be  commonly  called  one  flash, 


Sundry  Applications  of  Photography 

sidered  that  the  plate  is  probably  fogged  by  the  general 
illumination  of  the  sky  or  perhaps  by  flashes  in  other  direc¬ 
tions,  that  plate  is  lost,  and  another  must  take  its  place. 
As  soon  as  a  flash  comes  in  that  part  of  the  sky  repre¬ 
sented  in  the  camera,  the  lens  is  covered  and  the  plate 
removed  for  development.  It  is  not  satisfactory  to  allow 
the  plate  to  remain  with  the  lens  open  with  the  idea  of 
getting  the  picture  of  another  flash  upon  it,  because  a 
feeble  light  gaining  access  to  the  plate  after  the  image  of 
the  flash  has  acted  on  it,  is  very  liable  to  so  affect  the 
plate  that  the  flash  is  “  reversed  ”  and  appears  as  if  it 
had  been  black  instead  of  bright.  To  show  the  multiple 
character  of  flashes,  the  camera,  with  the  lens  open,  is 
continually  moved  from  side  to  side.  Each  constituent  of 
the  flash  passes  so  rapidly  that  the  movement  of  the 
camera  does  not  affect  the  sharpness  with  which  it  is 
depicted  upon  the  plate,  but  there  is  a  perceptible  interval 
between  each  constituent  and  the  next,  so  that  the  image 
of  the  next  falls  upon  a  different  part  of  the  plate.  By 
the  kindness  of  Dr.  H.  H.  Hoffert  we  are  able  to  give, 
facing  page  324,  a  copy  of  a  photograph  of  lightning  taken 
by  him  on  June  6,  1889,  by  swinging  the  camera  from  side 
to  side  as  he  held  it  in  his  hands.  It  shows  the  multiple 
character  of  at  least  three  separate  flashes.  The  most 
conspicuous  flash  is  shown  to  consist  of  three  consecutive 
discharges,  and  doubtless  there  was  another  befoie  these, 
the  image  of  which  did  not  fall  on  the  plate.  This  photo¬ 
graph  shows  also  that  the  air  remains  continually  lumi¬ 
nous,  at  least  in  parts,  during  the  intervals  between  the 
consecutive  discharges  which  we  associate  together  as 
a  single  flash. 

It  may  be  asked  how  is  it,  if  we  get  three  or  four 
distinct  images  on  the  plate,  we  do  not  see  the  multip  e 
character  of  the  flash  by  our  eyes,  why  cannot  we 
see  the  three  or  four  flashes  one  after  the  other  .  That 
is  as  we  have  explained  before,  because  we  see  nothing 

325 


Sundry  Applications  of  Photography 

for  less  than  about  the  one-tenth  of  a  second — the  impres¬ 
sion  produced  in  our  eyes  remains  for  that  time,  and  if 
the  second  flash  comes  before  the  impression  of  the  first 
has  died  away,  it  appears  to  us  to  be  a  continuous  effect, 
and  there  is  no  intermittency  so  far  as  our  eyes  are 
concerned.  Although  lightning  is  not  a  very  frequent 
phenomenon,  a  great  deal  has  been  done  to  elucidate  its 
character  by  means  of  photography. 

The  consideration  of  the  photography  of  phenomena  in 
the  sky  naturally  leads  us  to  think  of  those  objects  that 
are  far  beyond  the  clouds,  beyond  even  the  blue  sky,  and 
that  range  in  their  places  away  to  distances  that  it  is  im¬ 
possible  for  the  mind  of  man  to  conceive  of.  We  may  talk 
of  millions  of  miles,  of  thousands  of  millions  of  miles,  and 
millions  of  millions  of  miles,  but  we  cannot  comprehend 
such  distances.  Anyone  who  has  a  camera  and  knows 
how  to  use  it  can  photograph  the  sun,  moon,  and  stars, 
but  the  photographs  obtained  would  be  of  no  use  for 
astronomical  purposes.  The  image  of  the  sun  or  moon 
produced  by  such  a  lens  as  is  generally  used  with  a  half¬ 
plate  camera  would  be  about  the  sixteenth  of  an  inch  in 
diameter,  and  this  is  far  too  minute  to  be  of  service.  The 
moon  and  the  stars  being  far  inferior  to  the  sun  in  bright¬ 
ness  would  require  long  exposures,  and  if  the  camera  was 
fixed  they  would  show  as  streaks  or  lines  of  light  because 
of  their  apparent  movement  in  the  heavens.  For  practical 
work  therefore  we  must  have  a  camera  and  lens  that  will 
give  a  much  larger  image  and  that  will  move  constantly 
and  regularly  so  as  to  compensate  for  the  earth's  rotation. 
Such  an  arrangement  is  an  astronomical  telescope  with  the 
eyepiece  removed  and  a  holder  for  the  sensitive  plate  put 
in  its  place.  The  telescope  is  kept  moving  by  a  specially 
constructed  clock,  but  although  such  mechanism  is 
sufficiently  good  for  eye  observations,  it  is  not  perfect 
enough  to  keep  the  image  still  upon  the  plate  for  the  pro¬ 
tracted  exposures  that  are  sometimes  necessary.  A  smaller 

326 


Sundry  Applications  of  Photography 

telescope  attached  to  the  main  instrument  has  in  its  eye¬ 
piece  two  fine  wires  which  cross  each  other,  and  the 
observer  makes  by  hand  such  adjustments  as  may  be 
necessary  to  keep  the  image  of  the  required  star  exactly 
on  the  cross  wires. 

If  a  larger  image  is  required,  the  lens  used  must  have 
a  greater  focal  length,  for  the  diameter  of  the  image 
increases  exactly  in  proportion  to  the  focal  length  of  the 
object  glass.  And  if  the  focal  length  is  great,  the  diameter 
of  the  lens  must  be  increased  also,  because  we  must  have 
more  light  to  produce  a  larger  image  if  we  wish  to 
maintain  the  brightness  of  the  image.  Thus  astronomical 
telescopes  are  long  and  large,  and  the  size  of  the  larger 
instruments  is  limited  only  by  their  cost  and  the  mechanical 
difficulties  of  manipulating  them.  It  is  easy  to  see  the 
reason  for  this  with  regard  to  the  sun,  moon,  and  planets, 
which  give  images  of  measurable  size  with  the  smaller 
telescopes,  but  the  stars  are  only  points  of  light  to  even 
the  largest  instruments,  and  if  their  images  are  anything 
more  than  absolute  points,  it  is  because  of  the  optical 
phenomena  concerned  in  producing  the  images  and  the 
imperfections  of  the  lens.  If  then  we  have  mere  points 
of  light,  what  is  the  use  of  trying  to  get  a  large  image? 
The  size  of  the  image  in  this  case  is  not  the  image  of  an 
individual  star,  which  indeed  gives  no  real  image  at  all,  but 
of  the  little  patch  of  sky  that  is  being  dealt  with.  Ihe 
larger  the  image  the  more  space  will  be  shown  on  the 
plate  between  the  stars,  and  this  means  that  two  or  more 
stars  that  are  so  close  that  they  seem  to  be  joined  to  form 
one  star  when  using  a  small  telescope  will  be  separated 
by  a  larger  instrument.  When  dealing  with  objects  of 
small  luminosity,  such  as  stars,  nebulae,  and  comets,  the 
light-gathering  power  of  the  telescope,  which  is  represented 
by  the  size  of  the  object  glass,  is  of  great  importance. 
Doubling  its  area  is  equivalent  to  doubling  the  luminosity 
of  the  object,  and  thus  stars  that  are  not  bright  enough  to 


Sundry  Applications  ot  Photography 

be  visible  with  the  smaller  telescopes  are  brought  into 
view,  and  as  it  seems  probable  that  the  feebler  stars  are 
generally  less  bright  than  others  because  of  their  greater 
distance,  the  “penetrating"  power  is  increased  with  the 
light-gathering  power.  Every  telescope  has  its  fixed  limits 
in  this  direction  so  far  as  the  eye  is  concerned,  for  what 
an  observer  cannot  see,  he  will  not  be  able  to  see  by 
looking  for  a  longer  time.  But  in  photography  this  limit 
disappears,  for  the  plate  stores  up  the  effect  the  feeblest 
light  produces  in  it,  and  it  is  only  necessary  to  prolong  the 
exposure  sufficiently  to  get  registered  upon  it  objects  that 
we  can  never  hope  to  see  because  they  give  out  so  little 
light.  The  limit  here,  therefore,  no  longer  depends  upon 
the  telescope  directly,  but  upon  the  possible  increase  in 
the  duration  of  the  exposure,  or,  what  is  equivalent, 
increase  in  the  sensitiveness  of  the  plate  used. 

Astronomers  have  realised  the  advantages  that  photo¬ 
graphy  should  offer  them  from  the  earliest  days  of  the 
Daguerreotype,  and  with  the  advent  of  collodion  plates 
and  gelatine  plates,  the  applications  of  photography  in 
this  direction  have  steadily  increased  until  simple  eye 
observations  are  generally  of  secondary  importance.  The 
images  that  the  largest  telescopes  give  of  the  planets  are 
small,  about  perhaps  a  quarter  of  an  inch  in  diameter. 
It  is  rarely  if  ever  of  use  to  enlarge  them  because  the 
definition  is  not  good  enough.  The  difficulty  here  is  not 
so  much  instrumental,  but  because  of  the  constant  move¬ 
ments  of  the  air  and  the  different  temperatures  of  the 
various  currents  and  moving  masses.  As  air  varies  in 
temperature  it  varies  in  density  and  consequently  in 
refractive  power,  as  may  be  seen  by  looking  across  a 
chimney  top  from  which  hot  air  is  rising,  or  over  a  flame, 
at  objects  beyond.  This  moving  of  the  image  is  often 
sufficient  to  make  photographic  work  impossible.  Here, 
therefore,  a  practical  limit  to  the  duration  of  exposures  is 
often  imposed  upon  the  patient  astronomer.  But  in  spite 

328 


Sundry  Applications  of  Photography 

of  these  and  other  difficulties  Professor  Lowell  recently 
stated  of  photography  in  connection  with  Mars,  that  “the 
camera  has  shown  itself  capable  of  rising  beyond  the 
confirmatory  into  the  discovery  stage,  for  one  of  the  plates 
was  instrumental  in  the  detection  of  a  new  canal.  ’ 

There  is  one  difficulty,  which,  however,  might  have 
been  predicted  and  may  in  due  time  be  obviated,  namely 
that  the  image  on  photographic  plates  consists  of  metallic 
silver,  and  silver  is  affected  by  the  atmosphere.  The 
image  is  not  permanent.  We  see  that  silver  changes  by 
the  action  of  the  air  when  the  metal  is  in  mass,  as  articles 
for  domestic  use  need  constant  cleaning  to  remove  the 
tarnish  that  disfigures  them.  When  there  is  on  the  plate 
the  feeblest  deposit  that  can  be  seen,  as  in  the  images  of 
the  least  brilliant  stars  that  can  under  the  circumstances 
affect  the  plate,  we  cannot  be  surprised  that  the  minute 
amount  of  metal  present  should  suffer  change  throughout 
its  whole  mass  and  so  lose  its  visibility,  for  it  seems 
probable  that  any  change  from  the  blackness  of  the 
original  deposit  must  tend  towards  a  loss  of  density.  It  is 
probable  also  that  many  astronomers  regard  photography 
as  a  simple  mechanical  process  not  worth  consideration, 
and  that  they  do  not  pay  sufficient  attention  to  the  work 
to  get  as  nearly  as  possible  to  an  image  of  pure  silver  in 
clean  gelatine,  for  any  plate  in  which  these  conditions  are 
not  approximately  fulfilled  is  likely  to  contain  within  itself 
the  sources  of  its  deterioration.  This  is  not  a  matter  to  be 
lightly  set  on  one  side,  for  in  some  cases  it  has  been  found 
that  as  many  as  a  third  of  the  total  number  of  star  images 

have  disappeared  within  ten  years. 

In  the  consideration  of  lenses  we  saw  that  whenever 
light  is  bent  out  of  its  straight  path  by  passing  into  a 
second  medium  of  a  different  kind,  the  constituents  of  light 
are  bent  to  different  degrees,  so  that  the  light  is  separated 
into  its  parts,  as  in  the  rainbow.  This  is  a  difficulty  in 
lenses  which  has  to  be  overcome  as  far  as  possible,  but  by 

329 


Sundry  Applications  of  Photography 

taking  advantage  of  this  method  of  analysing  light  it  is 
often  possible  to  tell  the  nature  of  the  substance  that  is 
luminous,  and  for  this  purpose  it  does  not  matter  whether 
the  light  comes  from  a  lamp  on  the  laboratory  bench  or 
from  a  star  so  far  away  that  its  light  takes  hundreds  of 
years  to  travel  from  it  to  us.  By  this  method  of  light 
analysis  it  is  possible  to  tell,  not  only  the  nature  of  sub¬ 
stances  that  can  be  made  luminous  in  the  laboratory,  but 
also  what  substances  are  in  the  sun,  in  the  stars,  and  in 
comets  ;  not  in  the  planets,  for  they  shine  only  by  reflect¬ 
ing  the  light  of  the  sun,  as  the  moon  does. 

When  light  that  passes  through  a  small  hole,  or  prefer¬ 
ably  a  slit,  is  spread  out  into  a  band  of  colours,  red, 
orange,  yellow,  green,  blue,  and  violet,  the  result  is  called 
a  spectrum  (see  Fig.  5),  and  the  instrument  that  effects 
the  analysis  a  spectroscope.  A  full  or  continuous  spec¬ 
trum,  that  is  one  showing  all  the  colours  and  with  no 
gaps,  is  obtained  when  the  light  from  a  white  hot,  solid, 
non-volatile  substance  is  examined  with  a  spectroscope. 
But  volatile  substances  in  general  give  off  light  of  less,  often 
very  much  less,  complexity.  The  metal  thallium,  for  ex¬ 
ample,  when  heated  in  a  flame  gives  a  green  light  that 
cannot  be  decomposed.  However  much  the  light  is  bent, 
there  is  the  one  green  line  or  picture  of  the  slit  through 
which  it  is  admitted  to  the  spectroscope,  just  as  if  the 
light  had  not  been  bent  at  all.  This  green  light  is  simple 
and  cannot  be  decomposed,  and  thallium  is  the  only 
substance  that  produces  light  of  exactly  this  character,  and 
therefore,  when  such  light  is  obtained  we  know  that 
thallium  is  there  to  produce  it.  Similarly  sodium,  one  of 
the  constituents  of  table  salt,  gives  a  yellow  light,  which  in 
the  same  spectroscope  is  never  bent  so  much  as  the  green 
light  due  to  thallium.  Most  substances  give  very  complex 
lights  when  their  vapours  are  made  to  glow,  and  thpir 
spectra  consist  of  a  number  of  “  lines,"  or  images  of  the 
slit,  separated  from  each  other,  and  from  their  number 

330 


Sundry  Applications  of  Photography 

and  positions  the  substance  that  gives  rise  to  them  can  be 
identified,  for  no  two  substances  when  their  vapours  are 
luminous  have  ever  been  found  to  give  the  same  spectrum. 
In  this  way  we  know  that  the  sun  and  stars  contain  iron, 
sodium,  calcium  (the  metal  of  lime),  hydrogen,  and  numei  ous 
other  substances  of  which  our  earth  largely  consists.  And 
there  are  many  other  applications  of  this  method  of  light 
analysis  to  which  we  have  not  space  to  refer. 

In  eye  observations  of  spectra  it  is  obvious  that  we 
must  be  limited  to  light  that  affects  the  eye,  that  is  to 
visible  light.  But  we  can  photograph  not  only  all  the  light 
that  is  visible,  but  a  great  deal  more  that  extends  in  the 
spectrum  beyond  the  red  at  one  end  and  the  violet  at  the 
other.  Photography  here,  therefore,  at  the  same  time 
that  it  gives  a  permanent  record  that  can  be  measured  at 
leisure,  vastly  extends  the  range  of  the  work,  and  has 
brought  out  facts  that  could  never  have  been  known 
without  its  aid. 

In  an  ordinary  spectroscope  there  is  a  narrow  slit 
through  which  the  light  from  the  source  passes,  so  that 
the  constituents  may  be  separated  with  as  much  precision 
as  possible  and  without  overlapping.  But  suppose  that 
there  is  no  slit,  and  that  a  flame  is  coloured  with  both 
thallium  and  sodium  at  the  same  time,  we  can  get  an 
image  of  the  flame  as  easily  as  of  the  slit  by  suitable 
adjustment  of  the  instrument,  and  now  we  shall  get  a 
yellow  image  and  a  green  image,  side  by  side,  two 
coloured  images  of  the  flame,  the  one  produced  by  the 
yellow  light  of  the  sodium  and  the  other  by  the  green 
light  of  the  thallium.  The  two  lights  mixed  in  the  flame 
are  separated  as  completely  as  if  the  metals  had  been 
vaporised  in  two  separate  flames.  Now  the  sun  con¬ 
tains  many  substances,  and  by  isolating  the  light  from 
one  of  them  it  is  possible  to  photograph  the  sun  by  means 
of  that  light  alone,  and  so  to  ascertain  the  proportional 
distribution  of  that  particular  substance  on  the  surface 

331 


Sundry  Applications  of  Photography 

of  the  sun.  The  sun  has  been  photographed  by  means 
of  its  calcium  light,  and  where  that  light  is  bright,  there 
calcium  is  in  large  quantity,  and  where  it  is  dull  there  it 
is  in  smaller  quantity. 

Another  application  of  photography  in  spectrum  analysis 
is  in  detecting  double  stars  that  are  too  close  together 
to  show  as  two  distinct  stars  by  the  most  powerful  tele¬ 
scopes  ever  constructed.  This  might  seem  at  first  a 
hopeless  problem,  but  the  method  of  attacking  it  can 
be  made  clear  by  analogy.  When  a  whistling  engine  or 
car  passes  by  us,  the  whistle  giving  the  same  note  con¬ 
tinuously  as  is  usual,  the  note  is  quite  clearly  “lower'’ 
as  it  recedes  from  us  than  as  it  approaches  us.  It  gene¬ 
rates  sound  waves  of  the  same  length  all  the  time,  but 
as  it  approaches  us  each  successive  wave  starts  nearer  to 
us  than  the  preceding  wave  and  so  a  greater  number 
enter  our  ear  in  a  given  time ;  and  as  it  goes  from  us  each 
wave  starts  farther  away  from  us  than  the  preceding  one, 
and  therefore  we  have  fewer  in  the  same  time  than  if 
the  whistle  was  not  moving.  As  we  have  said  before, 
our  eyes  and  ears  know  nothing  of  what  goes  on  outside 
them,  they  are  affected  only  by  what  goes  into  them ; 
it  is  therefore  not  the  length  of  the  sound  wave  that 
concerns  us,  but  the  number  of  waves  that  we  receive 
in  a  given  time,  that  is  the  frequency  of  the  impulses. 
As  the  sounding  whistle  approaches  us  we  receive  the 
impulses  more  rapidly,  therefore  the  note  is  higher,  and 
as  it  goes  from  us  we  receive  them  less  rapidly  and 
therefore  the  note  is  lower  than  if  the  whistle  were 
stationary.  Exactly  the  same  thing  happens  with  light, 
and  if  two  stars  are  rolling  round  each  other,  when  one 
is  approaching  us  and  the  other  receding,  each  line  in 
the  spectrum  will  be  doubled,  the  two  new  lines  being 
one  on  each  side  of  the  position  of  the  single  line  that 
will  be  seen  when  both  the  stars  are  moving  at  right  angles 
to  the  line  of  vision.  The  line  that  is  displaced  toward 

332 


Sundry  Applications  of  Photography 

the  red  end  of  the  spectrum  will  represent  the  increased 
wave  length  caused  by  the  receding  star,  and  the  line 
displaced  towards  the  blue  end,  the  shortened  wave  length 
due  to  the  approaching  star.  Thus  the  spectrum  line 
gradually  divides,  the  two  lines  separate,  and  they  come 
together  again  to  form  one  line  according  to  the  move¬ 
ment  of  the  stars,  and  the  period  of  revolution  and 
the  rate  at  which  the  stars  are  moving  towards  us  or 
from  us  can  be  calculated  from  the  displacement  of  the 
line. 

But  it  is  impossible  to  describe  the  various  applica¬ 
tions  of  photography  adequately  without  writing  a  treatise 
on  each  of  the  subjects  concerned.  We  must  therefore 
be  content  with  mentioning  only  a  few  other  subjects 
with  which  it  is  inseparably  connected.  Stereoscopic 
views  show  objects  in  their  full  solidity  by  giving  each 
eye  that  view  of  the  object  that  it  would  receive  if  the 
solid  object  were  being  looked  at.  The  difference  in  the 
view  as  seen  by  the  two  eyes  is  often  very  small  because 
the  points  of  view  of  the  two  eyes  differ  by  only  a  little, 
they  are  so  near  to  each  other.  It  would  be  hopeless 
by  hand  drawing  to  represent  these  differences  with  any 
approach  to  sufficient  accuracy,  but  here  photography  is 
perfect.  And  the  stereoscope  is  not  a  mere  plaything 
or  interesting  toy ;  a  stereoscopic  view  is  to  a  single 
photograph  just  the  difference  between  binocular  and 
monocular  vision  if  the  photographs  are  properly  made  : 
it  reveals  the  true  shape  of  things,  and  the  comparative 
distances  of  things  or  of  their  various  parts.  It  is 
applicable  to  microscopic  objects  as  well  as  to  larger 
views,  and  even  the  moon  has  been  subjected  to  similar 
treatment  by  photographing  it  in  two  different  aspects, 
for  a  pair  of  stereoscopic  views  may  be  made  from  the 
same  point,  if  the  object  is  turned  a  little  so  as  to  present 
the  same  appearance  that  it  would  have  from  another 
point  of  view.  The  stereoscopic  photographs  of  the  moon 

333 


Sundry  Applications  of  Photography 

show  it  as  it  would  appear  to  an  enormous  giant  with 
eyes  a  vast  distance  apart. 

In  making  observations  with  scientific  instruments 
it  often  happens  that  the  field  of  the  instrument  is  but 
feebly  luminous,  and  that  the  adjustments  of  it  alternate 
with  the  reading  of  a  scale.  The  scale  must  be  rather 
brightly  illuminated  that  its  fine  divisions  may  be  seen, 
and  the  constant  changing  of  the  eye  from  a  dull  to 
a  bright  object  not  only  strains  it  but  may  impair  the 
accuracy  of  the  observations.  Cameras  have  been  made 
to  photograph  the  scale  whenever  a  reading  is  desired, 
and  this  not  only  spares  the  eye  and  facilitates  the 
adjustments,  but  gives  a  record  free  from  bias  or  possible 
misreading.  Sixty  readings  can  be  recorded  on  a  small 
plate,  so  that  the  expense  for  photographic  materials 
is  negligible.  Similarly,  cameras  have  been  designed  for 
photographing  gas,  water,  and  electric  meters,  watchmen's 
clocks,  &c.,  and  for  the  continuous  and  automatic 
recording  of  the  readings  of  meteorological  instruments, 
such  as  barometers  and  thermometers. 

We  began  our  consideration  of  this  subject  by  stating 
that  photography  is  writing  or  drawing  by  means  of 
light.  We  have  seen  how  almost  every  thing  and  every 
movement  can  be  recorded  and  often  investigated  by  its 
means,  and  how  it  is  possible  in  some  cases  to  deal  with 
and  investigate  the  light  itself  as  well  as  the  objects  that 
the  light  may  illuminate.  In  leaving  light  we  are  really 
going  away  from  photography,  yet  we  may  refer  to  the 
use  of  photographic  methods  in  other  connections,  for 
the  general  method  of  dealing  with  photographic  plates 
is  the  same  whether  they  have  been  affected  by  light  or 
by  any  other  force  that  produces  a  similar  change  in 
them.  Light  so  changes  the  silver  salt  of  a  gelatino- 
bromide  plate  that  a  developer  is  enabled  to  remove  the 
bromine  and  leave  the  metal  silver  as  direct  evidence  of 
the  change.  The  silver  salt  is  rendered  amenable  to  the 

334 


Sundry  Applications  of  Photography 

action  of  the  developer  by  other  agencies  than  light.  It 
was  by  putting  a  piece  of  a  uranium  mineral  near  to  a 
plate  and  leaving  it  there  for  some  time,  that  it  was  dis¬ 
covered  that  the  mineral  affected  the  plate,  and  this 
led  eventually  to  the  discovery  of  radium.  Since  then 
a  great  many  other  substances  have  been  found  that 
give  out  a  something,  either  rays  similar  to  light  rays, 
or  gases,  or  something  of  the  kind,  that  affect  a  photo¬ 
graphic  plate.  It  seems  that  so  common  a  substance 
as  potassium,  which  is  the  essential  constituent  of  potash, 
gives  out  something  of  this  sort.  The  photographic 
method  of  testing  for  these  emanations  is  not  the  only 
method,  but  it  has  the  great  advantage  that  an  exceed¬ 
ingly  feeble  action  may  be  detected  by  simply  leaving 
the  substance  to  be  tested  near  to  a  plate  in  the  dark 
for  the  necessary  time,  and  this  may  be  extended  as 
may  be  necessary,  for  the  action  is  cumulative.  If  we 
compare  the  millionth  of  a  second,  which  is  sufficient 
exposure  to  the  light  of  an  electric  spark  to  affect  a 
plate,  with  the  months  that  may  be  necessary  for  a 
potassium  salt  to  produce  a  similar  change,  we  get  some 
idea  of  the  extreme  feebleness  of  the  potassium  com¬ 
pound  in  this  direction  and  might  suppose  that  it  is 
self-luminous  to  that  extremely  small  extent.  But  what¬ 
ever  it  is  that  is  given  off  by  potassium  salts  and  sub¬ 
stances  that  act  similarly,  it  will  pass  through  various 
opaque  media,  and  presumably  is  not  what  we  generally 
understand  as  ‘Might." 

It  must  not  be  supposed  that  forces  are  similar  because 
they  can  produce  one  similar  effect,  and  it  does  not  follow 
that  because  radium  glows  and  affects  a  photographic 
plate  as  light  does,  that  other  forces  that  affect  the  plate 
are  comparable  to  light  in  a  general  sense.  The  Roentgen 
rays  certainly  affect  the  plate  and  might  perhaps  be 
supposed  to  be  a  kind  of  light,  but  every  known  kind  of 
energy  is  able  to  render  the  silver  bromide  less  stable  and 

335 


Sundry  Applications  of  Photography 

therefore  amenable  to  the  action  of  the  developer.  Heat, 
mechanical  force  such  as  pressure,  electrical  energy,  and 
the  contact  of  substances  which  appear  to  act  in  a 
chemical  way,  will  all  render  silver  bromide  developable 
under  suitable  conditions,  and  it  seems  not  impossible  that 
these  effects  may  eventually  be  utilised  as  the  action  of 
light  has  been.  One  fact  to  bear  in  mind  is  that  the 
relationship  between  the  photographic  plate  and  light  is 
not  so  exceptional  as  it  used  to  be  thought  to  be,  and  that 
it  is  possible  that  we  are  only  on  the  threshold  of  the 
applications  of  methods  that  we  at  present  associate  almost 
exclusively  with  light.  But  looking  at  photography  even 
as  it  is,  we  do  not  hesitate  to  say  that  the  growing  import¬ 
ance  of  it  from  every  point  of  view,  educational,  com¬ 
mercial,  and  scientific,  is  not  realised  as  it  should  be,  or 
photography  would  not  be  left  to  a  few  specialists,  and 
a  comparatively  large  number  of  those  who  regard  its 
practice  merely  as  a  pastime  or  an  amusement. 


336 


INDEX 


Names  of  persons  are  indexed  only  when  they  are  very  definitely  associated  with  some 
result  or  process,  or  are  connected  with  the  early  history  of  photography. 


Aberration,  Chromatic,  60 
Aberration,  Spherical,  44 ,  58 
Aberrations  in  lenses,  52 
Absorption  of  light,  232,  243 
Accelerators  in  development,  152,  155 
Acetone  as  accelerator,  155 
Achromatic  lenses,  62 
Actinometers,  144 

Additive  method  of  three-colour  colour 
photography,  253 

Aeroplanes,  Photography  from,  322 
Albumen  film  on  glass,  83,  89 
Albumen  silver  prints,  185 
Albumenised  paper,  185 
Alchemists,  72 
Ammonia  as  accelerator,  155 
Ammonia  in  intensification,  173 
Ammonium  persulphate  as  a  reducer, 
169 

Analysis  of  movement,  286 
Anastigmats,  67 
Aniline  printing  process,  209 
Animals,  Photography  of,  295 
Animated  photographs,  287 
Aplanats,  66 
Apochromatic  lenses,  62 
Arago,  80 

Archer,  F.  Scott,  84,  89 
Artificial  light  for  portraiture,  313 
Asphalte,  78 
Astigmatism,  59 
Astronomical  photography,  326 
Auroras,  Photography  of,  324 
Autochrome  plates,  257 


Bacon,  Roger,  71 

Balloons,  Photography  from,  321 


Barbaro,  Daniello,  72 
Becquerel’s  colour  photography,  246, 
248 

Bennett,  Charles,  104 
Birds,  Photography  of,  295 
Bitumen,  78 
Black  art,  96 

Black  to  white,  The  range  from,  146 
Bleaching  action  of  light,  34,  70 
Bleaching-out  process  of  colour  print¬ 
ing,  262 
Blue  prints,  197 
Bolton,  W.  B.,  98 
Bromide  paper,  Preparation  of,  179 
Bromide  printing,  179 
Brougham,  Lord,  76 
Bullets,  Flying,  Photography  of,  284 


Calotype,  83 
Camera,  The,  131 
Camera,  Adjustments  of  the,  133 
Cameras,  The  earliest,  71 
Cameras,  Early,  112,  132 
Cameras  for  photogrammetry,  319 
Cameras,  Panoramic,  320 
Carbon  printing,  198 
Carbon  tissue,  199 
Changes  produced  by  light,  30,  34 
Chemical  changes,  33 
Chevalier,  C.,  7 9 
Chromatic  aberration,  60 
Chromo-lithography,  252 
Cinematography,  287 
Cinematography  applied  to  the  micro¬ 
scope,  308 

Cinematography  in  natural  colours,  288 
Cleaning  glass  plates,  107 


Index 


Clearing  baths,  162 
Clouds,  Measuring  the  height  of,  323 
Clouds,  Photography  of,  322 
Coating  gelatine  plates,  107 
Collodion,  90 

Collodion  dry  plates,  97,  IOI 
Collodion  positives,  95 
Collodion  processes,  84,  89 
Collodio-chloride  papers,  187 
Collotype,  214 

Colour,  Absorption  of,  232,  243 
Colour,  Analysis  of,  231 
Colour  blindness,  234 
Colour  depends  upon  the  light  used, 
230,  233 

Colour,  Effect  of,  in  photomicrography, 
305 

Colour,  Effect  and  control  of,  228,  241 
Colour  filters,  233 
Colour  of  silver  prints,  18 1,  183 
Colour,  Photography  of,  245 
Colour  screens,  233 
Colour  sensitiveness  of  colour  sensitised 
plates,  238 

Colour  sensitiveness  of  ordinary  plates, 
235 

Combined  toning  baths,  189 
Continuing  action  of  light,  202 
“Control,”  165,206 
Control  of  light,  36 
Copper  chloride  in  intensification,  17 1 
Correct  exposure,  129 
Cost  of  photography,  1 1 1 
Criminals,  Photography  of,  313 
Cross-lined  screens,  221 
Curvilinear  distortion,  55 


Daguerre,  L.  J.  M.,  79 
Daguerreotype  process,  80 
Dallmeyer’s  rapid  rectilinears,  66 
Dark  ground  illumination,  305 
Dark  light,  Photography  by,  242 
Dark-room,  Illumination  of,  243 
Davy,  Sir  Humphry,  76 
Deceptive  designs,  265 
Detail,  Emphasis  of,  in  photomicro¬ 
graphy,  305 
Detective  cameras,  280 
Developable  condition  produced  by 
other  means  than  light,  335 
Developable  image,  83,  85 
Developer  stains,  161 
Developer,  The  requisites  of  a,  152 
Developers,  Various,  154 


Developing  machines,  160 
Developing  P.O.P.,  190 
Development,  149,  157 
Development  by  time,  1 59 
Development,  Early,  78,  83 
Development  of  bromide  prints,  180 
Development  of  carbon  prints  by  diges¬ 
tion,  203 

Development  of  wet  plates,  93 
Development,  The  image  produced  by, 
161 

Digestion,  Development  of  carbon 
prints  by,  203 

Disappearance  of  star  images  on  photo¬ 
graphic  plates,  329 
Distortion,  Curvilinear,  55 
Distortion  due  to  the  flatness  of  the 
plate,  270 

Double  anastigmat,  67 
Double  stars,  Detection  of,  spectro¬ 
scopically,  332 

Drops  of  water,  Falling,  Photography 

of,  284 

Dry  plates,  97,  IOI,  104 
Dusting-on  process,  208 
Dyed  plates,  238 


Early  photography,  70,  73,  76,  &c. 
Efficiency  of  shutters,  279 
Electric  light  for  portraiture,  313 
Emanations  that  affect  photographic 
plates,  335 

Emulsions,  Collodion,  99 
Emulsions,  Gelatine,  103 
Engineering  works  photographic 
records,  317 

Error  in  photography,  264 
Etched  plates  by  Niepce,  78 
Etched  plates  for  photo-mechanical 
purposes,  21 1 
Ether,  Luminiferous,  23 
Exposure,  The,  131 
Exposure,  Conditions  that  affect  the, 
140 

Exposure,  B'ffect  of,  123,  128 
Exposure  in  bromide  printing,  180 
Exposure  meters,  144 


Factories  for  plate  making,  106,  1 1 1 
Facts  and  photography,  266 
Fading  of  negatives,  329 
False  images,  273 
Ferric  chloride  as  a  reducer,  169 


Index 


Ferric  oxalate  as  a  reducer,  169 
Ferricyanides  in  intensification,  173 
Ferroprussiate  process,  197 
Ferrotype,  96 

Ferrous  oxalate  in  intensification,  172 

Field  of  a  lens,  53 

Films  instead  of  glass,  112 

Filters,  Colour,  233 

Finders,  281 

Finger-prints,  313 

Fixing,  160 

Flare  spots,  274 

Flash-lights,  283 

Flying-machines,  Photography  from, 

322 

Formic  aldehyde  as  accelerator,  155 
Frequency,  Wave,  28 


Gallic  acid,  153 
Gas-light  papers,  183 
Geber,  72 

Gelatine  emulsions,  103,  108 
Gelatino-bromide  dry  plates,  105 
Gelatino-chloride  papers,  187 
Ghost  images,  273 
Glass,  Early  use  of,  88 
Glass  for  dry  plates,  107 
Glass-works  at  Jena,  63 
Globe  polish  as  a  reducer,  168 
Goerz  double  anastigmat,  67 
Gold  toning,  188 

Growing  plants,  Photography  of,  285 
Gum-bichromate  printing  process,  206 
Gun  cotton,  90 


Half-tone  blocks,  Preparation  of,  222 
Hand  cameras,  281 
Herschel,  Sir  John,  82 
Horn  silver,  72 

Hunt’s  colour  photography,  247 
Hurter  and  Driffield,  118 


Illumination  of  the  object  in  photo¬ 
micrography,  303 

Image  produced  by  development,  161 
Image  production,  38 
Infra-red,  Photography  by  means  of 
the,  242 

Instantaneous  photography,  276 
Intensification,  166,  170,.  175 
Interpretation  of  photographs,  265, 
266 


Inversion,  Lateral,  87 

Invisible  stars,  Photography  of,  328 

Isochromatic  plates,  236 

Jena  optical  glass  works,  63 


Kites,  Photography  from,  322 


Large  image  lenses,  294 
Large  photographs,  309 
Latent  image,  83,85 
Lead  ferricyanide  in  intensification, 
I74 

Lens,  Function  of,  48 
Lenses,  Aberrations  in,  52 
Lenses,  Achromatic,  62 
Lenses,  Apochromatic,  62 
Lenses  for  telescopes,  microscopes,  and 
cameras,  compared,  49 
Lenses,  Large,  Advantages  of,  50 
Lenses  old  and  new,  52 
Lenses,  Rectilinear,  58 
Light,  17 

Light  affects  colour,  230 
Light,  Analysis  of,  28,  231,  330 
Light,  Changes  produced  by,  30,  34 
Light,  Control  of,  36 
Light,  Estimating  the  intensity  of,  142 
Light  filters,  233 

Light,  Its  action  on  gelatino-bromide 
plates,  123 

Light,  The  rate  at  which  it  travels,  23 
Light  waves,  26 

Lightning,  Multiple  flashes  of,  325 
Lightning,  Photography  of,  324 
Line  blocks,  Preparation  of,  218 
Lippmann’s  colour  photography,  249 
Lithography,  78,  215,  252 
Living  pictures,  287 
Luminiferous  ether,  23 


Maddox,  R.L.,  103 

Magnification,  Useful,  Practical  limit 

of,  301 

Marey’s  photographs  of  athletes,  Sic., 
286 

Mercuric  chloride  in  intensification,  17 1 
Mercuric  iodide  in  intensification,  175 
Metallography,  307 
Microphotography,  309 
Microscope  objectives,  49,  299 


339 


Index 


Misrepresentation  by  photography,  267 
Mist,  its  cause  and  effect,  240,  242,  323 
Monckhoven,  Dr.,  105 
Motion,  Photography  of,  284 
Moving  animals,  Photography  of,  286 
Muybridge’s  photography  of  moving 
animals,  285 


Natural  colours,  Photography  in, 
246 

Negative,  The  perfect,  164 
Negatives,  86 

Niepce  de  Saint  Victor,  83,  89 
Niepce,  J.  N.,  78 


Objectives  for  telescopes,  micro¬ 
scopes,  and  cameras,  compared,  49 
Oil  prints,  207 

Oil  prints  from  bromide  prints,  208 
Omnicolore  colour  plates,  257 
Optical  glass  works  at  Jena,  63 
Orthochromatic  plates,  236 
Orthochromatic  results  on  ordinary 
plates,  23S 
Over-exposure,  129 
Ozobrome,  204 
Ozotype,  203 


Paget  prize,  105 
Panchromatic  plates,  236 
Panoramic  cameras,  320 
“  Papier-velours,”  206 
Particles,  Smallest  photographable, 
3°6 

Particles,  Smallest  visible,  304 
Penetrating  power  of  telescopes,  328 
Perfect  negative,  The,  164 
Permanency  of  prints,  192,  195,  203 
Persistence  of  vision,  277 
Petzval’s  portrait  lens,  65 
Photo-etched  plates,  218 
Photogrammetry,  318 
Photographic  surveying,  318 
Photographs  made  without  man’s 
intervention,  36 
Photographs  on  apples,  37 
Photography,  Early,  70,  73,  76,  &c. 
Photography  of  colour,  245 
Photography  v.  Painting,  17 
Photogravure,  223 
Photo-lithography,  215 


Photo  -  mechanical  colour  printing 
methods,  261 

Photo-mechanical  printing,  21 1 
Photomicrography,  299 
Photomicrography,  Early,  76,  82 
Photo-physiological  studio,  286 
Photo-zincography,  215 
Physical  changes,  32 
Pigeon  post,  310 
Pigment  printing,  198 
Pinatype,  260 

Pinhole  gives  an  image,  40,  71 

Plain  paper  prints,  185 

Plants,  Growing,  Photography  of,  285 

Plate,  The,  1 1 5 

Plate-making  factories,  106 

Plates,  Action  of  light  on,  123 

Plates,  Prices  of,  1 1 1 

Platinum  image,  Permanency  of  the, 

*95 

Platinum  in  intensification,  175 

Platinum  paper,  Preparation  of,  194 

Platinum  printing,  193 

Playertype,  190 

Poitevin,  198 

Porta,  G.  B.  della,  72 

Portrait  lenses,  65 

Portraiture  by  artificial  light,  312 

Positives,  86 

Positives,  Collodion,  95 

Potassium  bromide  as  retarder,  1 56 

Pouncy,  206 

Prices  of  plates,  in 

Printing  in  silver,  178 

Printing-out  papers,  184,  187 

Printing  plates  by  Niepce,  78 

Prisms,  47 

“  Profession,”  Photography  becomes  a, 
81 

Protar,  67 

Prussian  blue  prints,  197 
Pyrogallic  acid,  153 
Pyroxyline,  70 


Radium,  335 

Rainbows,  Photography  of,  324 
Rapid  rectilinears,  66 
Rapid  symmetrical,  66 
Reade,  Rev.  J.  B. ,  82 
Record  Association,  National  Photo¬ 
graphic.  3 1  5 

Records,  Photographic,  314,  316,  &c. 
Rectilinear  lenses,  58 
Reduction,  166,  168 


Index 


Reflex  cameras,  281 
Refraction,  45 

Resolving  power  of  lenses,  300 
Retarders  in  development,  152,  x56 
Reticulation,  214 
Retouching,  176 

Reversal  by  excessive  exposure,  125, 
129 

Reversal,  Lateral,  87 
Rollable  film,  1 14 
Roller  slides,  113 
Ross’s  rapid  symmetrical,  66 
Roentgen  rays  affect  photographic 
plates,  335 

Sayce,  B.  J.,  98 

Scale  of  reproduction,  Adjustment  of, 
292,  296 

Scales  of  instruments  photographed 
instead  of  reading  them  by  inspection, 

334 

Scheele,  C.  W.,  75 
Schultz,  J.  H.,  73 
Screens,  Colour,  233 
Screen-plate  process  of  colour  photo¬ 
graphy,  255 

Seebeck’s  colour  photography,  246, 
248 

Self-toning  papers,  189 
Sensitisers,  184 

Sensitiveness,  Estimation  oh  1 17 

Sensitiveness  got  by  ammonia,  105 

Sensitiveness  got  by  heating,  104 

Sensitiveness  not  constant,  1 1 8 

Sensitiveness  of  the  plate,  1 1 5 

Sepia  platinum  prints,  195 

Shutters,  277 

Silver  bath,  92 

Silver  chloride,  72 

Silver  cyanide  in  intensification,  173 

Silver  intensification,  174 

Silver  nitrate,  73 

Silver  nitrate  as  secret  ink,  74 

Silver  printing,  178 

Silver  prints,  The  life  of,  192 

Sky,  Photography  of  the,  322 

Snapshotting,  282 

Soap  bubbles,  Bursting,  Photography 
of,  284 

Soda,  Washing,  as  accelerator,  155 
Sodium  sulphite  in  development,  1 5 7 
Spectroscopy,  33° 

Spectrum,  28,  231 
Spherical  aberration,  44>  58 


Stains,  Developer,  16 1 
Standing  waves,  248 
Stannotype,  213 

Stars,  Double,  Detection  spectroscopi¬ 
cally  of,  332 

Stars,  Invisible,  Photography  of,  328 
Stationary  waves,  248 
Steinheil’s  aplanats,  66 
Stereoscopic  work,  333 
Studios,  Portrait,  138 
Substratum,  107 

Subtractive  method  of  three-colour 
colour  photography,  258 
Sulphur  toning,  181 
Surveying  by  photography,  318 

Talbot,  W.  H.  Fox,  81 
Tank  development,  160 
Tannin,  153 

Telegraphic  transmission  of  photo¬ 
graphs,  226 

Telephotographic  lenses,  294 
Telescope  objectives,  49i  64 
Temporary  support  in  carbon  printing, 

201 

Thames  colour  plates,  257 
Three-colour  methods  of  colour  photo¬ 
graphy,  251 

Time  methods  of  development,  1 59 
Tintypes,  96 

Toning,  Combined,  baths,  189 
Toning  platinum  prints,  195 
Toning  silver  prints,  181,  186,  188 
Transferring  in  carbon  printing,  200 
Transfers,  216 

Trisodium  phosphate  as  accelerator, 

156 

Truth  in  photography,  264 


Ultra-microscopy,  304 
Ultra-violet,  Photography  by  means  of 
the,  242 

Under  exposure,  128 
Uranium  compounds  affect  photo- 
graphic  plates,  335,  .  . 

Uranium  ferricyanide  in  intensification, 

x73 

“  Uto  ”  paper,  262 


View  lenses,  66 

Visual  reading  of  instruments  replaced 
by  photography,  334 


341 


Index 


Vogel’s  discovery  of  colour  sensitising, 
235 

Wave  frequency,  28 
Wave  length,  26 

Wave  length,  Effect  of,  in  resolution, 
306 

Waves,  Light,  26 
Wedgwood,  Thomas,  76 
Wet  plate  process,  92 
Wheel  of  life,  284 


When  to  use  colour  sensitised  plates, 
239 

White  to  black,  The  range  from,  146 
Woodbury  type,  212 
Works  photographs,  317 


Zeiss’s  protar,  67 

Zenker’s  theory  of  colour  photography, 
247 

Zincography,  215 
Zoetrope,  284 


THE  END 


Primed  by  Ballantynb,  Hanson  Co. 
Edinburgh  London 


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